Well, the oven trick worked. I soaked the assembly for an hour at 200F, and it was still reeeeally tight but the old pedal came off ok.
It's interesting. Nothing is cross-threaded or stripped, but it is just really really tight. To the point where I'm slightly nervous about putting the new pedal in- it feels just as tight. Don't want to damage anything, but I guess I got the old one in there without hurting anything...