Will this be available later for DVR 625 or PVR 721 or are they going to offer and exchange/upgrade to a different receiver
Like the man says, NO. Anyway, those units have USB 1.1 and external storage requires USB 2. And they won't likely offer an upgrade for such an optional feature. However, I expect in 2 or 3 years, as they want to move SD to MPEG-4, they may offer an upgrade.
I have a ViP622, a 721 and a 508. I'm happy with all three. I planned on waiting until they pry those 2 SD (grandfathered) DVRs from my fingers before giving them up. However, if my bedroom SDTV departs this world sooner, and it's looking like it might, and I replace it with, say, a 40" or so LED backlit LCD, which I might (if price is right), then I'll get a ViP722 and move the ViP622. And it's bye bye for the 721 and 508.
YMMV. But if you like this feature, it may be time to consider upgrading on your own. Charlie would like that. But you'd also get to enjoy more features, rather than doing without, outwaiting him.