I had a fairly big custom Python 2 static website generator ( github.com/csplib/csplib ), which I'd about given up transfering to Python 3, after a couple of aborted attempts. My main issue was that the libraries I was using didn't have Python 3 versions.

AN AI managed to do basically the whole transfer. One big help is I said "The website output of the current version should be identical", so I had an easy way to test for correctness (assuming it didn't try cheating by saving the website of course, but that's easy for me to check for)