You said that migrating the existing forums would be the most painful part of a forum software change?
Most of these forum web apps have fairly straightforward “thread” and “comment” tables which can be read and written with with a simple Python SQLAlchemy script. I can volunteer to do that, but can’t guarantee that the password hash methods any two web applications use will be identical. Forcing people to change passwords through a “forgot” link should be easy, though.