This was my approach to a large rewrite I'm working on. First create the multiple layers of test suite to map out existing behavior at the unit, API, and integration level (something that should have been done incrementally over the 20 years this software has existed), then predicate the new code on the tests. It's been remarkably effective.

I plan to eventually get there, just need to find the time. It’s a lot of code, and a lot of it is not set up for testability.