What is a Rewrite Project
A rewrite project is a type of software project where a team builds a new version of an existing product based on the old product. This often happens when the original product has become hard to maintain, slow to scale, or outdated in terms of technology or user experience. The goal of a rewrite is usually to deliver the same value as the old product, but with improvements such as more features, fixed existing bugs, better performance, cleaner architecture, and a more modern and user-friendly interface.
How Testing Is Different in a Rewrite Project
In a rewrite project, the main goal of testing is to make sure the new version works the same way as the old one, while also delivering the planned improvements. Testers spend more time comparing version 1 and version 2 to ensure that it does not miss any features or logic. Even small differences in calculations, workflows, or error handling can create serious problems if they go unnoticed.
Unlike traditional projects, testers in a rewrite must check both new requirements and existing behavior at the same time. They are not only asking, “Does the new feature work?” but also, “Does this still work the way it used to?” and “Does this work better than the old one?”. Rewrite projects often aim to make the system faster, more responsive, and easier to use. Testing needs to confirm that these improvements actually help users and do not accidentally disrupt familiar processes.
During the testing process, testers should also look for features that need to be improved or enhanced. Because of this, testing in a rewrite often feels like detective work. Testers first need to understand how the old system truly behaves in real life, and only then can they verify that the new version matches it or improves on it in the right way.
Difficulties for Testers in a Rewrite Project
One of the biggest challenges in a rewrite project is missing or unclear behavior from version 1. Legacy systems often lack complete documentation, and some behavior exists only because of historical decisions or quick fixes. Testers may discover edge cases that are not written anywhere but are still used by real customers.
Another difficulty is the changing and evolving requirements. Over time, business needs shift, and a rewrite may try to fix old mistakes while adding new functionality. Testers are facing many changes in requirements and business logic during the testing process. This makes it harder for testers to decide whether a difference between v1 and v2 is a bug or an intentional improvement.
In addition, some features in version 1 may never have been fully completed or properly tested. Testers must confirm whether to reproduce these incomplete features, improve them, or remove them entirely. These require close collaboration with BA and stakeholders, adding complexity to the testing process.
Similarities with Testing in a Normal Project
Despite these differences, rewrite projects still share many similarities with normal software projects. Testers still follow core testing principles such as planning, execution, regression testing, and defect reporting. Test cases, test automation, and continuous integration pipelines remain essential parts of the workflow.
Testers still need to collaborate closely with developers, designers, and product teams to understand requirements and clarify expectations. Quality standards, release criteria, and risk assessments continue to apply, even if the context is more complex.
Some Useful Tools
There are several useful tools that can help make testing in a rewrite project less challenging. Lighthouse is a valuable browser extension that allows testers to evaluate and compare the performance and UI/UX of version 1 and version 2. By using Lighthouse reports side by side, testers can gain a clearer perspective on whether the new version actually delivers meaningful improvements over the original product.
Another helpful tool is GitHub Copilot, although its use requires approval from the client and product manager. When permitted, GitHub Copilot can be a powerful assistant for analyzing and exploring the version 1 source code. It helps testers and developers uncover hidden logic, edge cases, and existing behaviors that are not clearly documented in the acceptance criteria, reducing the risk of missing critical functionality during the rewrite.
Conclusion
Testing in a rewrite project is not just about finding bugs, it is also about protecting trust. Users expect the new version to feel familiar while being better, faster, and easier to use. This balance makes testing more challenging than in a normal project, but also more impactful. By understanding the differences, recognizing the difficulties, and applying proven testing practices, testers play a critical role in turning a risky rewrite into a successful evolution of an existing product.