No, it isn't possible to remove the AC unit without releasing the refrigerant, but first, check to see if there's any there to begin with! Lots of older AC systems have bled all the gas off years ago. Just push the stem of either the high or low side charging valve a little and see if you get a squirt of pressure. It looks like the valve on a tire stem, and works the same way. If there is still pressure, the gas can be bled off into a discarded refrigerant tank, if you have a gauge manifold to connect to the system. If you don't have the equipment to do that, your local air conditioning shop would probably be glad to have some recycled R-12 gas, so call a repairman and see if he will purge the system for you. That stuff is really expensive now, and it can be recycled.
Once the system is drained, be sure to plug the lines and components to keep air and moisture from getting into the lines and parts. It ruins the receiver-dryer in particular in just a few hours of exposure to the atmosphere.
Jerry
