Sean,
Yes, they could have it at two locations. But if they put it on 61.5 (east coast) and 148 (west coast), consumers expect that service. For example, if Dish were to add ESPN-HD or Hdnet to 61.5 (or 148), there would be a lot of new customers installing dishes to receive that satellite and programming. So Dish can't very well say in July, "we're going to remove that programming next month." That would royally piss customers and installers off. Any HDTV programming on 61.5/148 now will have to stay there for a long, long time.
They may want to use 61.5/148 for other programming eventually; if they want to migrate customers off 61.5/148 to 121 over time, obviously adding more HDTV to 61.5/148 will not suit that goal. At some point, they may want to use the bandwidth on 61.5/148 for other programming, or they may want to sell their license for that spectrum to someone else like Cablevision, which seems at least conceivable with the recent [effective] purchase of a KU-FSS capacity at 105.
The 121 spectrum is in the KU-FSS band, and it will be delivered by the Echostar IX satellite, which they intend to launch in May (IIRC). Because the FSS satellite band represents a higher frequency, and because it will operate in closer proximity to other FSS band satellites, it will require a larger dish. Echostar is claiming that a 26" dish will be sufficient to receive all channels from 110, 119, and 121. So you would be replacing your 20-21" Dish500 with a 26" "Super Dish," and throwing out the second dish pointed at 61.5/148.
If 121 is the slot for all future HDTV (and remember, this is still speculation), then it will be interesting to see how they do the dish upgrade / replacement. I suspect they would include the new 26" dish with the forthcoming 921 HD PVR receiver. And it would likely be offered free (with free install ?) to existing customers that subscribe to a new HDTV channel package at 121.