Anyway, I don't know if the new MPEG4 receivers will support legacy or not. So far, everything they have made supports legacy, so.............When it comes to the LNB, it doesn't matter other than the fact that DP is more convienent.
When the new receivers arrives I expect they will be downward compatable with legacy LNBs.
MPEG4 is also downward compatable meaning its supports MPEG2 (and MPEG1) compression as well.
The big question is what satellites will HD reside on. Presently it's mostly on 110, 61.5 and three channels on 148. If they move them or spread out over more satellites you may be forced into a major upgrade.