Arg. I have been dealing with my AC system for the last 3 weeks. I have a lot of information LOL, and my AC is STILL not working
Anyways, in order to change out to 134 [completely] you have to change everything except the actual lines and condensor/evap coils: Compressor, dryer, o-rings, fittings, oil. There are probably 15 o-rings in all.
NOW, that being said, many people simply pull a vacuum on the system, put some adaptor fittings on it, and refill it with R134a. This may or may not work in all cases and definitely is not the "proper" way to do it, but it has been proven to work in some cases.
If you do convert over to the GenIII compressor, please post, as I may be doing the same. The dryer is probably the hardest part to get to on a stock car, as it is right behind the front bumper. Not sure but you may be able to pull the driver's side wheelwell liner and access it.
I ran into the problem of even after the fact that I have my license (I'm an HVAC engineer, but don't know much about cars) and can get R12 replacement freon (called Hot Shot) I can't pinpoint an R12 leak somewhere in my system. They don't sell dye kits for R12 anymore, R134 dye kits are not compatible, and a "sniffer" is $200 friggen dollars. For now I'm sweating like a pig.....100* and 75% relative humidity down here