Oh I forgot, you can external Serial ATA the drive. Meaning, you can get an eSata enclosure and use the old drive in the new unit. It however, will disable the current drive. It does not add on to it. So as long as you are not adding new recordings, you can use the old drive, drain it, and then shut off, unplug the drive, and have it boot to the one in the unit. Of course, that is if you are allowed to view recordings done on another unit. That of course will reset all your preferences like Season Passes. Also, what are you supposed to do with existing receiver? If you have to send it back, opening the case violates the warranty.
Bottom line now that I think more about it, keep both units going for now and drain the old drive.