well I had the same dilema as you, except I also had a 501 in a sspare room. When the 921 price dropped to $549 I went ahead and purchased. DishDepot is crediting me $100 on the return of my 811 (which is $1 more than I paid for it).
So I kept the 721 and now have the 921 along side in the TV room. I am very happy with it. So far I have not had any issues with MY 921. I will maintain my 721 with all of it's original timers, just in case, but have also programmed all of the HD show timers on the 921. I'd love to also sell the 721 but the 921 just doesn't have enough storage for me all by itself.
So now it's HD on 921 and SD on the 721...with the 721 also ready to catch any overflow as needed.
In short, this setup works great for me. Now just waiting for the 921 to get OTA guide, although even this is not a showstopper since the SD network channels show up in the guide.