Damn, I've been waiting since a dealer told me in mid-2004 that we'd see 70+ HD nationals later THAT year.
My bet is that the mpeg2's remain mpeg2. There are too many folks who still have HD-TiVO's, plus the recent agreement w/TiVO in support of those units. Also, taking channels away from some people is a great way to attract a class-action suit or action by Congress.
I highly doubt they would expend extra bandwidth by mirroring mpeg2-available channels in mpeg4.
Couple of things - first, if you didn't know - DISH just did this same thing with VOOM - they moved it on to MPEG4 and notified the subs that were getting it that they had to change to new receivers - so do't be sup[rised if DirecTV does it also, because it makes sense.
Second - they most likely WILL put all of the current HD offering onto MPEG4 right away (and mirror the MPEG2 channel). They are in fact going to migrate the remaining MPEG2 HD subs over to tyhe new platform.