If there is no charge left in the system, change out the o-rings, drier,clean the evaporator and condenser with a flush, pull a vacuum and let it sit. I usually throw a charge of Nitrogen into the system and let it sit a while to verify that there are no leaks. If there are none, I'd say pull the vacuum again and have it filled with R12 as it's more efficient and your hoses and system is sized for that refrigerant. If you want to convert to 134A, you'll need to change out a bit more than the O rings to retrofit. As long as these systems hold their charge, it's worth trying to stick with the original R12. But if you really want to think long term or have doubts, or want to do it all yourself, may as well change over.