You can use a different card than what was originally in the receiver if both receivers and both cards have a clear balance and released from the account. I believe you would have to speak to a special department to have this done though.
A retailer maybe but you wont get anything for your 510 as a trade in unless you find a crazy retailer. 501, 508, 510, 522, 721, 921, and 942 pvr/dvr's are no longer produced and no longer sold by dish network.
Call Dish Network and give them the smart card from the 301 and the receiver number from the 510 and see if they can get it activated. If the receiver does not have some recent software then they cannot do this. Make sure you put the smart card into the receiver first and that the 301 is turned off and released from the account.
Also, try and get into the "tech support" CSR queue, as this is something the normal CSR probably doesnt do everyday. An email to firstname.lastname@example.org might also help if you explain what you are trying to do.
The only way this will work is if the accounts the receiver and card were associated with are clear with no balances.