Yes, you can call Dish and get your old receiver added as a second Dish receiver. You could have done that at anytime actually. I believe the cost is still $5 a month for the second receiver. Also, why take down the SA dish? Echostar now owns all of that spot and I would imagine will be using it for something. You might want to wait to see what they put up before you junk it.
Of course you would need some handy dandy switchboxes or whatever they are called and if your Dish receiver is new and your SA receiver is old, then the old receiver may not like the switches or lnb's or something on your new setup. Found this out as they tried to upgrade my dish after a huricane, but couldn't get my legacy receivers to work with the new dish or switch. If you don't have dish 500, you could just point the old dish at 119 or 110 and get something without using switches, but you would still have to pay the cost for the second receiver.
Hope this helps.