The MPEG-4 HD network upgrade is designed to be as gradual as possible. The conversion schedule article outlines when each channel will be upgraded.
When an HD channel is upgraded, the SD version may still be available in MPEG-2, so you won't be without your channels until you have upgraded to an MPEG-4 compatible receiver.
Once a channel has been upgraded to MPEG-4, the channel can only be viewed on an MPEG-4 compatible satellite receiver. You will see a message in your Guide indicating that particular channel has been upgraded, and a blank screen if you tune to that channel's MPEG-4 feed from an MPEG-2 receiver.
If you already have an MPEG-4 compatible satellite receiver (compatible models are listed above), you won’t experience any impact or notice any difference when a channel is upgraded from MPEG-2 to MPEG-4.