My 501 remote controls my 921 just fine. Only thing missing is the PIP controls, and the never-used "R" button. Several other Dish IR remotes will also work.
I use mine via an IR repeater to control the 921 from the other end of the house where I have a feed from the 921 SD output. Oh - the 501 "TV/Video" button is the same as the 921 "SD/HD" button.