What's your test coverage? It's usually not that hard to reach 100% if you have a decent coverage analyzer. Or even if you don't - one easy way is to never write a line of code unless you have a failing test that exercises it.

Changing a function signature is the kind of thing that tests can & should catch. You need them anyway to catch edge cases in the logic and document the code, and then once you've done that you usually get pretty good coverage for free.