They won't have MPEG-4 versions of SD channels if those channels are available in HD. If and when they go to all MPEG-4, they'll simply go with the HD versions for all subscribers.
If there is no hd version, then you'll have mpeg4 version in sd, which WILL happen. There's tons of LIL channels that will never be anything other than sd. No point in wasting time upconverting a sd only and forever channel to hd just for mpeg4 when you can send it sd mpeg4 and let the box convert it and not waste the bandwidth