This in essence makes the HR21 the actual upgrade that it was meant to be. It doesn't have much different on the inside from the HR20, except that they didn't put ATSC tuners in it to keep costs low since they'd have to pay licensing fees per box. By adding the upgrade box, you get the dual ATSC tuners back, integrated programming guide and you don't have to use the TV's ATSC tuners with the annoyance of switching inputs, etc. We really like our HR21 except for the fact that we don't get the local channels in HD so therefore have to switch to the TV antenna input and it makes it difficult to keep track of two games at once.
Supposedly this will also work with the H21 after a while when all the software/firmware upgrades get finalized and pushed out.
I'm not a big fan of having the additional box since the HR20 had the tuners built in but at least they were smart enough to make it stackable so it's easier to keep things looking good.