I just figured this out today and thought I'd share since I haven't seen any posts pointing it out.
I've had a 921 for a while and just purchased an MX-500. I couldn't get the MX-500 to learn the 921 remote codes because the 921 remote only sends signals to the reciver in UHF format. I found this user guide on the Dish web page today: (http://www.dishnetwork.com/downloads/pdf/technology/remote/5_6_8_Remote_Control.pdf)
It basicly says that the Dish 5.0, 6.0, and 8.0 remotes are the same. However, there is one major difference. The 5.0 uses IR only to control the receiver, the 6.0 uses IR OR UHF to control the receiver, and the 8.0 uses only UHF to control the receiver. So, I got a used 6.0 remote from EBay for about $10 and used that to program my MX-500. A 5.0 remote should work as well.