I'm not sure about the HVAC world, but in automotive circles freon is "re-used" all the time. With the proper equipment, recover/recycle machine, it can be done easily. The main problem I think you will run into in this situation is getting the freon out of one unit and into the other. The system is under pressure, correct? So if you hook up a tank to the system and open the valve, the presure will equalize between the system and the tank. So the volume of the catch tank will determine how much freon you can actually recover because without applying vacuum to the system it will never get completely empty. Now another problem is if there is any mositure in your catch tank if so how are you going to get it out? If water/moisture gets in to the freon it can (most likely will) ruin your compessor because water DOES NOT compress!