Agreed, whether you rewrite or change in-place you're not going to be confident in the outcome unless you have tons of end to end tests first. Then you need to

Agreed, whether you rewrite or change in-place you're not going to be confident in the outcome unless you have tons of end to end tests first. Then you need to find and skip/ignore the tests that don't actually test end-to-end behavior or business logic, just some implementation detail along the way. Only then will you be free to do what is probably the first step if you have a crappy codebase: start establishing domain boundaries and interfaces and splitting things across those lines.