This is an interesting question that I also wonder about. I believe that as long as you don't cancel your service you could add a receiver and drop the other one. The only exception I can think of would be the DHA (Dish Home Advantage or something like that) plans.
I'm waiting for my Dish Pro Adapter to install my Dishplayer 7100 that I had from a couple years back. For some reason we didn't qualify for a "free" Dishmover promotion and had canceled several years ago but had returned just under a year ago (so we are still in our commitment phase with our 301).
That's an important point for you. The Dishplayer 7100/7200 are not Dish Pro (DP) LNB or Switch compatible. If you have a newer system then you are most likely DP for your Dish 500 and will need either a Dish Pro Adapter (adapts one receiver) or if you intend to add more receivers or sats a Dish Pro Plus 44 switch (DPP44) that includes a built-in Dish Pro Adapter.
The receiver was fanastic for me before we canceled our earlier service, but had a few bugs. Recent topics on this site show that many have had great improvement with the current software for the Dishplayer. This is the reason I'm bringing it out of the closet and getting ready to reactivate the service. The Dishplayer is also the only Dish DVR product that will allow "unofficial" end-user hard drive upgrades for more recording time.