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.
Well, the will remain MPEG2 for a while, perhaps a year or so, but they will be shutdown as quickly as they can so they can free up the bandwidth on the main birds. This will allow them to increase SD PQ (that'll be the day) or add more SD channels or expand interactive stuff.
There is no wasting of bandwidth on the new sats. ALL HD will be on them.
And there are not that many people with just HD-Tivo's. At their peak there were only 250K of them. The rate that you see people getting MPEG4 equipment (even those with HR10s) is very fast (just check the Tivo forum). Only a very small amount of these people will still not have any MPEG4 receiver come this time next year, most will have at least added an HR20 to their receiver mix.
And there is no extra "agreement" with Tivo. There always has been one. Tivo providing an update next year is simply part of the contract that has always been there since it was signed a couple years ago. Nothing more then that. There are some 4 million SD DirecTivos and those aren't going away any time soon until the hardware dies.