You can replace the 921 HDD, but you'll need to use Norton Ghost to copy the OS from the original.
There are 4 partitions on the 921 HDD. You only need to clone the 3 small ones.
Ghost can also backup the 3 small partitions to PC files (once the 921 HDD fails completely there's nothing to clone).
Sound like a lot of hassle? Dish would probably send you a replacment 921 for not must more then the price of the HDD. Call em and ask.
Now, you have to decide if it's worth keep the 921 alive. You probably have around 2 years before all Hi-Def goes to mpeg4.
The 921 could still be a nice replacment for a 50x or a 621. But it's not gonna hold a candle to any of the NBR receivers (522/625). But, since they are only giving back $100 for the 921 it might be work keeping (you could always keep the HDD and throw the rest away).