Just my two cents:
DirecTV went with Ka because they had it, and they put the HD on Ka instead of Ku, because if you're rolling out a new service, you put it on the new frequencies. Would have been monumentally stupid to try to put HD on Ku and move SD to Ka, necessitating a total swap of the entire install base. There's no reason HD can't (and won't) be on Ku eventually, frequency band and encoding (not modulating) schemes are completely independent.
And you'll know when DirecTV is about to start the transition, it'll be around two years after the date they stop installing new 101-only dishes and SD only set tops. As long as they're still installing 101 only dishes or SD only set tops, then the transition is a very long way away. They might still get away with 101-only dishes, if they were going to do a flash cut, but they will have to replace every single SD only set top box with an HD capable receiver, even if it's downconverting for an SD TV set, before they can move to all MPEG-4. Because at that point, what would be the point of having MPEG-4 SD only boxes and duplicating all that bandwidth. If you're going to distribute all those MPEG-4 boxes, might as well make them capable of decoding and downconverting HD and then you can drop half the encoders at the broadcast center, open up a TON of bandwidth, reduce the number of boxes you support, eliminate an installation call when a customer upgrades their TV, etc.