I've done the big rewrite a few times, and what I've learned from it is that a system so "bad" that it is in need of a rewrite does not have a caretaker who und

I've done the big rewrite a few times, and what I've learned from it is that a system so "bad" that it is in need of a rewrite does not have a caretaker who understands it well enough to provide perfect documentation of choices made and reasoning behind them. In fact, a system that bad typically has plenty of choices which were made for no good reason at all. So, you never go into the big rewrite with perfect documentation or good tests, because if you had those you wouldn't need a big rewrite.