Yip, you can't really do a re-write unless you've gone to the bother of understanding what is there in the first place (and wrapping tests around it, if at all possible).

And if you're going to the trouble to understand it and wrap tests around it, then maybe refactoring the existing codebase is a smarter choice. If less sexy.

I think a good mindset to have is to look for the challenge in making decent refactorings.