Unit tests are exactly for code that is often rewritten, because it ensures that whatever interface still behaves the same, regardless of the implementation. This a large portion of the point of unit tests: not for testing the initial implementation but confirming that any subsequent implementation behaves the same.

Using AI to write Unit tests is one of the few use cases I somewhat understand, but even that turns out horrible with improper supervision. I reviewed one Pull Request once where the testing was so horribly cobbled together and nonsensical that I rewrote those tests by hand (after asking the person I was reviewing to fix it twice and them only making it worse by letting their AI rewrite them)