This is why we have the strangler pattern, right? You wrap the legacy functionality with a set of tests that define the functional requirements of the code, the

This is why we have the strangler pattern, right? You wrap the legacy functionality with a set of tests that define the functional requirements of the code, then your new refactored solution needs to keep those tests passing. (And then once you've done that, make sure that your "hacks and bugfixes" get test cases so you can make sure the next refactor accounts for them!)