While an option that lets you continue on the same course without changes would be ideal, I do wonder if it might be reasonable to go the Steam route and just have an "original style" patch available from outside of Steam? Or would that just cause the same kind of problems as Patreon is giving people, I wonder.
Either way, hopefully it won't come to that in the first place.
...As an aside, "no modded Koikatsu" is among the weirdest policies I've ever heard of, especially given that Steam seems to (mostly) have no issues with either of the two components of that premise on their own.