I agree, but not having an interface is a great way to make sure you probably are never able to swap out the implementation (even if someone came along and wanted to do all the work).

Drastic fixes and Rewrites almost always consist of three things:

1. Writing tests to ensure functionality

2. Writing an interface or shimming the existing functionality to redirect control flow

3. Implementing and testing the rewritten functionality/improvements

Obviously you can't do it everywhere, but life is much better when (2) was done up front.