Comment by andoando

If your refactoring includes changes to interfaces, different abstractions, logical changes, business logic, then most of your tests need to be effectively rewritten.

The only part where I see unit tests being useful for refactoring is making changes to the internals of a single unit. Its always been more trouble than its worth for me.

In some cases it makes sense, like testing small units that heavy in logic (function that calculates order prices for example, scientific computing, etc). But unit testing every single piece of code has always seemed dogmatic to me (unit tests are good engineering, write unit tests always everywhere). Everything has tradeoffs and as engineers I think our job is to understand the pros and cons and apply them effectively.