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Backup 921 pvr before 622 arrives?

Why would you loose it? Unless it does a 921 hickup you should be able to leave the recordings on there for future viewing.
 
You could also leave the 921 hooked up to another input on your TV and watch the recorded programs only until you've gotten through them all. I did this when I replaced my wife's 721 with a 942. If your TV only supports one set of HD inputs, you'd have to swap the cables to toggle back and forth or hook the 921 up via 480p component or s-video, The 921 won't need to be hooked up to satellite in order to play back recorded programs,