How to unfasten electrical subpanel attached with ramset
Powder-actuated gun nails are extremely hard. You don't want to have to cut them with a saw, and you probably won't have room anyway. 

Get yourself an angle grinder with an abrasive wheel. Bracing your hands well against the box, carefully grind away just the heads of the nails. You should then be able to pull the box off the nail shafts. 

Obviously, take all necessary precautions when working around electrical current, and use eye and ear protection.

Caveat: Be sure that you have enough length available on your primary conductors to make the move you have planned. You may also be required to install conduit behind the box, where the wires are exposed.