I went through the same process. Two could be replaced with 2.x native functionality so just got ditched and the one I still needed didn't have a 2.x upgrade path so I replaced it with one that has turned out to be a definite improvement.
Eek, yeah. That's going to be a serious PITA. Haven't run forums on that scale so can't really help but good luck. Hopefully some of the pros around here can give some advice.