Default to incremental replacement over a full rewrite. Rewrites underestimate the behavior encoded in the old system and force you to maintain two things while shipping nothing new. Prefer strangling the system piece by piece behind a stable interface, with every step delivering value on its own. Consider a full rewrite only when the platform is genuinely unsupportable and you can protect the timeline from feature pressure.
Why interviewers ask this
Interviewers use this to test engineering judgment under commercial pressure. They want awareness that rewrites are the classic way to lose a year, a preference for incremental migration, and clarity about the narrow conditions where a rewrite really is correct. They also want to hear how you handle the team's motivation, since the urge to rewrite is usually about pain that has cheaper cures.
How to structure your answer
- State the default (incremental replacement) with the reasoning.
- Ask what problem the rewrite is actually meant to solve.
- Describe how you would slice it so each step ships value.
- Give the conditions under which a full rewrite is correct.
Example answer
My default is no, and then I try to understand what they actually want, because rewrite is usually a symptom. Sometimes it is genuinely a dead platform, and sometimes it is that the test suite takes forty minutes and every change is terrifying, which is a much cheaper problem to fix. When replacement is right I want it incremental: put an interface in front of the old system, move one capability at a time behind it, and make sure each slice is releasable and delivers something on its own. That way the project survives a reprioritization, which a two year big bang rewrite never does. On my last team we replaced a legacy order pipeline that way over about nine months, and we paused it twice for business reasons without losing anything. The case where I would do a real rewrite is when the runtime or framework is out of support with no incremental path, and even then I want an explicit written conversation with leadership about the feature freeze, because the classic failure is a rewrite plus a full roadmap.
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See how it worksFollow-up questions to expect
- How do you keep the team motivated during a long migration?
- How do you know when the old system can finally be deleted?
- What do you do if the migration is half done and priorities change?
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