SOLVED THE MOTHERFUCKER.
There are some problems with data and sync between the legacy and modern systems that I need to solve to keep it solved, but that's trivial. Tomorrow, I have to rip out a LOT of debug code, put in fixes to keep the data correct, and commit.
Finished work ~1930. Kinda done for the evening. Going to take a shower to cool off, go to bed, and catch up on my RSS feeds.