As mentioned, the first step is to create some tests that can validate the current behavior, so that changes don't break things.  This is a non-trivial task.  I know from experience with a similar refactoring that even seemingly simple changes can have unexpected consequences, and that getting good functional test coverage (not code-line test coverage) is hard.  This is complicated by the fact that regrtest is an over-evolved mess.  My ideal is to move appropriate pieces of the functionality into unittest and make regrtest a wrapper around that, but obviously I haven't spent much time actually doing that.

I don't think that regrtest tests need to be run as part of the standard python test run, by the way, though I suppose they could be.