Well, the FIRST thing to go will be distant HD networks...
Sunday ticket and existing channels have COULD go Mpeg4, but they don't have anywhere to 'mirror' them during the transition, so likely they won't change them until they get a new ConUS satellite up.
PROBABLY what willl happen is they will start releasing new HD channels next year in Mpeg4, if you want them you'll have to upgrade. If your happy with the channels you currently get, then they'll probably work for a year or two. Once most people upgrade on their own to get new hot channels, then they won't care so much about the handfull of TiVo fans leftover..
I think Sunday ticket will be Mpeg2 next year as they won't have any new sats for it by football season, but 2007 will be mpeg4 probably...