Legacy systems can be impossible to integrate with. They can make CI/CD tooling a nightmare. And, they can put your organization at risk of major cybersecurity failures, system outages, and user frustration.
Refactoring legacy code is an important software modernization strategy that dev teams use to update an entire system or application, or just its most risk-prone parts.
Your code might be a big, jumbled mess of dependencies. You might be struggling to wrangle a monolith. Or maybe, your code was built using a framework that, no matter how hard you try, just isn’t the right fit for the system.
Whatever the cause, refactoring might be the best modernization approach.
Keep reading to learn more about what refactoring is, how it’s done, and how to choose the right development partner for your modernization project.
What is refactoring legacy code?
Refactoring legacy code is when a software development team updates the structure and architecture of outdated code, but doesn’t change the functionality. When a team adds or removes features, that is considered rewriting or rebuilding.
Refactoring code is more cost-effective than rebuilding a system from scratch, but there are limits. If the underlying product no longer meets user needs, or if the codebase is so tangled that testing and future development are blocked, then a rebuild may be the smarter long-term move. Refactoring is the right path when the core functionality is still valuable and the system just needs to be modernized for better performance, maintainability, and scalability.
Why refactor legacy code (top benefits)
Refactoring legacy code offers the potential to cut back on massive development expenses while paving the way for future innovation that will please customers, improve internal efficiency, or whatever else you’re hoping to achieve.
Cost savings – Maintaining legacy systems is expensive. You have to pay for the upkeep of outdated technology, and new development is often more expensive because you can’t get things done quickly. By refactoring and modernizing your code, you can cut down on unnecessary costs.
Adhere to modern best practices – Following current best practices isn’t something you do just for the sake of it. When you use modern technologies, you can attract better development talent, while keeping your current developers happy and retaining more of your staff. And when your code is modern, you can implement other best practices, like continuous deployment, to spread those advancements to other teams and be a competitive player in your market and a competitive employer.
Improve scalability – Outdated systems are prone to breakage, especially during times of heavy user load. You can improve your application’s scalability by refactoring the code—which leads to fewer system outages and customer support hassles.
Pave the way for future innovation – One of the top motivations for refactoring legacy code is to unlock organizational potential.
Improve interoperability – Outdated systems are typically not interoperable. You can’t easily integrate legacy systems with new systems, meaning you can’t implement modern apps and services. When you refactor your code, you can rearchitect the system so that it can be integrated with the platforms that matter to your organization. This, in turn, can save you hundreds of thousands of dollars, because you can integrate with other software, rather than having to build all of the new functionality you need from scratch.
The bottom line is this: when your code is outdated or organized in a monolithic structure, developers can be worried about making changes and avoid them or feel frustrated with how dangerous and breakage-prone any updates are. This can affect everyone internally in your organization as well as your end users.
Refactoring allows you to breathe new life into your application with code that is easier to support.
5 steps of legacy code refactoring
The steps for refactoring legacy code depend on whether or not you need to modernize the entire application or make a change to just one area of the code to handle a new query. However, most organizations are looking to modernize the system as a whole so they can proactively address application risk.
Step 1. Audit the system’s architecture, code, and UX
Start with a full audit of the legacy system. This includes:
Mapping the current architecture
Assessing the structure and quality of the code
Evaluating the UX for usability gaps
Even if you’re only planning to refactor part of the application, this step is non-negotiable. Refactors often uncover tightly coupled logic, outdated dependencies, or UX friction that wasn’t obvious from the outside. What begins as a small update can quickly turn into a broader code cleanup—or reveal the need for architectural changes.
Step 2. Do deep product strategy
If your system is outdated, you might have been failing to do product strategy as well. Does the system still meet user needs? Is the product roadmap current?
For systems that are still actively maintained, you might not need to adjust your refactoring approach. But, if your application hasn’t undergone product strategy in months or years, then you might discover that there are so many required functionality changes that refactoring won’t be enough, and you need to rebuild the application instead.
If you're unsure where your product stands, consider running a discovery sprint. This short, focused effort helps uncover user needs, validate assumptions, and determine whether a refactor or rebuild is the right path forward.
Consider bringing in a fresh set of eyes. DevSquad offers a discovery sprint workshop to assess your system and help define the right path forward—no matter who handles the implementation.
Step 3. Determine the new architecture
Your system’s architecture should be simple, efficient, and scalable. It needs to process data efficiently today, and be ready to meet the needs of your users a year from now.
The above steps should affect the new architecture that you choose and help establish an application modernization roadmap. Make sure you have a clear understanding of where the system stands today and where you want it to go. Consider all your options before deciding on an architectural pattern.
Some of the most common options include:
Staying with a monolith, but simplifying and modularizing it for better maintainability
Modular monolith, where the codebase is still unified but broken into clearer domains and boundaries
Microservices, where features are split into independently deployable services
Serverless, where certain functions are offloaded to cloud services to reduce infrastructure overhead
Event-driven architecture, for systems that benefit from asynchronous communication between services or modules
Many dev teams blanket-apply microservices as the new architecture to deal with monoliths, but microservices can cause development and testing challenges so be sure to talk with experts that offer application modernization services before deciding.
Step 4. Refactor the code
During this step, you extract logic code to maintain an application’s existing functions and behaviors and then rewrite the code, using more modern frameworks and simpler approaches.
It’s important to always remember that refactoring is not starting over. It’s a reshaping of existing code to improve structure, clarity, and maintainability. The goal is to retain the same behavior while reducing complexity and tech debt.
If you just need to update a small part of your app, you might use the “application seams” method described in one of the expert tips below. With this method, you essentially wrap the change in a clean API so you’re not causing a trickle-down effect of bugs.
For larger updates, use characterization tests to lock in current behavior before making changes. This gives you a safety net while you refactor legacy code effectively and makes the refactor less risky.
Step 5. Run performance and QA tests
With the right system architecture and refactored code, you should now be able to properly cover your application with unit tests. Write manual test cases as well as automated testing scripts for every functional unit of the product.
Add automated regression testing to your pipeline to make sure you’re not reintroducing old bugs during future changes. Pair this with automated functional testing to validate that every feature still works end-to-end. Following automated regression testing best practices helps reduce risk and speed up releases.
Make sure to also have some QA testers manually explore the app in an ad hoc manner, so that they can find bugs that arise during more user flows.
With refactored code, your performance tests should yield much better results. Be sure to test the performance of your app in a variety of environments, such as OS, browser, battery life, etc.
You should also test the interoperability of the system. Test any one-way or two-way integrations to be sure that data is flowing as expected.
If you're looking for a more scalable testing process, DevSquad offers automated testing services as part of ongoing product development. We help teams implement the right balance of functional and regression tests to keep shipping fast—without sacrificing stability.
Refactoring versus other modernization strategies
Legacy systems need to be modernized to avoid massive business risks. But refactoring isn’t the only approach. In fact there are about 6 popular approaches available to you when modernizing legacy systems.
Below, we see how refactoring stacks up against the other most popular options for custom software modernization:
Refactoring code versus rewriting code
Refactoring code is a completely different process from rewriting code. When rewriting code, you’re essentially trashing all of the old code and writing it again from scratch. This can be too time-consuming and cost-prohibitive for many organizations, particularly if the system or application is complex and robust.
But with refactoring, you improve the structure of the old code without changing its functionality. This is more cost-effective, but requires more of a commitment to QA testing, as there’s an increased risk of bugs and software crashes.
Refactoring code versus rebuilding the system
When rebuilding a system, you’re not just refactoring or rewriting old code—you’re reimagining the functionality.
There’s a good chance that if your technology is out of date, so is your product strategy. You might need to reconsider all of the systems’ functionality. What features are unnecessary? What features can be removed, combined, or redesigned? What new features need to be added? What other systems should your modernized platform be able to integrate with?
Depending on the nature of your software modernization project, you might refactor the code for some features, while completely rebuilding others.
How to know if refactoring isn’t the right approach
Can you refactor your code? Or do you need to rewrite the code or rebuild the system?
These are the top signs that refactoring alone won’t be enough:
No one on your team knows how to maintain the code
The code is slow and inefficient
It’s nearly impossible to add new functionality or fix bugs
The code doesn’t function or perform how you expect
The code isn’t actively maintained and developed
4 expert tips for refactoring code
Refactoring is complex business. Keep this expert advice in mind.
1. Be mindful of behavior changes
Michael C. Feathers authored the most popular book on dealing with legacy code in 2004. Although some aspects are outdated, much of the strategy mentioned is still applicable. He reminds us that whether we call something a bug fix or a new feature, what really matters is how it will affect the user experience. It’s super important to consider how refactoring your code could impact the required behaviors from users before proceeding.
“There is a big difference between adding new behavior and changing old behavior. Behavior is the most important thing about software. It is what users depend on. Users like it when we add behavior (provided it is what they really wanted), but if we change or remove behavior they depend on (introduce bugs), they stop trusting us.”
– Michael C. Feathers, author of Working Effectively with Legacy Code
2. Try the seam method for small refactoring fixes
Kevin Ball describes a refactoring project where the team just needed to change the way user roles were created and formatted. A simple task, and yet one that was at the core of the product and led to many dependencies. He used a process that he calls “application seams” to make the change without introducing functional changes that would then affect other features.
“Identify a location where you can create a clean API. This often involves ‘hoisting’ or ‘wrapping’ low-level functionality that is being accessed directly up into a higher level service or class. Next, refactor the service to create this API, introducing no functional changes. This often includes writing a bunch of unit tests for the new API. These unit tests serve the dual purpose of verifying your current implementation and making it safe to introduce changes in the layers underneath the API. Finally, update all calling code to use the new API. This should also introduce no functional changes, and to the extent possible, be validated by type checking and unit tests.”
– Kevin Ball, Engineering Manage at Humu
3. Separate refactoring from other development for a safer approach
Chris Birchall reminds us that refactoring shouldn’t be dangerous. So long as we’re not taking on too many changes or combining refactoring with other code changes, we should be able to safely refactor.
“When performed correctly, refactoring should be perfectly safe. You can refactor all day long without fear of introducing any bugs. But if you’re not careful, what started out as a simple refactoring can rapidly spiral out of control, and before you know it you’ve edited half the files in the project, and you’re staring at an IDE full of red crosses. Separate refactoring from other work. Splitting refactoring work and other changes into separate commits in your version control system also makes it easier for you and other developers both to review the changes and to make sense of what you were doing when you revisit the code later.”
– Chris Birchall, author of Re-Engineering Legacy Software
4. Centralize refactors to maximize leverage and avoid backsliding
When dealing with large, stubborn codebases, the team at Stripe has learned that spreading out a refactor across multiple teams often leads to inconsistent progress and missed goals. Jake Zimmerman discussed this in detail at the 2024 QCon in San Francisco. Specifically, Jake noted that centralizing the migration within a single, experienced team leads to better automation, deeper expertise, and a higher chance of completion. This approach is only effective, however, if it includes two things: a clear point of leverage and a good ratchet.
“A centralized migration is going to take fewer engineering hours overall, but it's going to be spread across a longer time horizon. While that refactor is happening, there's going to be plenty of chances for our work to just accidentally be undone unless there's some mechanism in place to prevent backsliding. Specifically, it's not enough to just have some way to ratchet incremental progress, it has to be a good ratchet.”
— Jake Zimmerman, Tech Lead of Sorbit at Stripe
Leverage gives a small team the power to enact meaningful change across the entire system. Ratchets prevent backsliding by enforcing incremental progress—ideally in a local, automated, and actionable way. Without both, long-running migrations risk getting undone or abandoned before they ever finish.
How to choose the right development team to help you refactor your code
If you plan on hiring a consulting firm to manage refactoring, you need to make sure you choose the right team.
Here are some of the most important factors to consider:
Expertise in modernization (not just new development) – Make sure that the team you work with has successfully modernized and refactored code in the past.
Delivers product strategy – Look for a team that offers product strategy, so you can be sure you’re creating a product that satisfies user needs in the best possible way.
Fully managed team with all of the resources you need – The team should manage the project end-to-end with all of the necessary talent and resources, including frontend developers, backend developers, product managers, tech leads, UX designers, DevOps engineers, and QA managers.
Can take ownership or handover the product – The team should be able to match your long-term goals for product ownership, whether that’s taking ownership of the product for the foreseeable future or helping you upskill your team and managing the handover in an efficient and professional manner.
Refactoring legacy code can be done unit by unit to protect current functionality while still paving the way for improved performance and interoperability.
Ready to refactor your code and unlock your potential? Learn more about DevSquad.
Frequently asked questions (FAQs)
What is legacy code?
Legacy code is any codebase that’s difficult to maintain, test, or extend—regardless of how old it is. It often lacks proper test coverage, relies on outdated dependencies, or is tightly coupled in ways that make change risky. Even systems that are still in production and generating revenue can be considered legacy if they slow development or increase operational risk.
How do I know if I am working with legacy code?
You’re likely working with legacy code if developers are hesitant to make changes, bugs are hard to isolate, or small updates cause unexpected failures. Other signs include poor test coverage, outdated frameworks, limited documentation, and difficulty integrating with modern tools like CI/CD pipelines or third-party platforms.
What are the main challenges with managing legacy code?
Legacy code is expensive to maintain and risky to change. It often slows development, increases the likelihood of outages, and creates security vulnerabilities. Teams may struggle with tightly coupled logic, outdated dependencies, and limited test coverage, making even minor updates feel dangerous and frustrating for developers and end users alike.
How do I know if refactoring my legacy code is the right approach?
A discovery sprint is where you should start. A deep dive into what you have is the only way to really know the path forward. Refactoring is the right approach when the core functionality of your system is still valuable, but the code structure is holding you back. If performance, scalability, or maintainability are the main issues then there is a good chance refactoring is the right path forward.
Should I perform legacy code refactoring in-house?
In-house refactoring can work if your team has deep system knowledge, time to focus, and experience with modernization. However, many teams bring in outside experts to reduce risk, accelerate progress, and gain an objective perspective. A consulting partner like DevSquad can lead strategy, refactoring, and testing—or support your team through a structured handoff.