Okay, word of advice before upgrading:
Take all of your characters out of custom maps and out of the Cape of Distant Worlds. This will prevent you from losing them in the case that something goes horribly wrong.
MM is working fine for me, though I'm not sure what I did to make it work. I did have to uninstall (as in remove EVERYTHING from the FearlHeart folder manually) and reinstall the game several times, so maybe that did it... somehow. I dunno. >.<
EDIT: Oh! And if you're really concerned about keeping a character, make a record of its size, marking, and colors so you can easily remake it in case something goes really, REALLY wrong.