If I were doing a conversion from R-12 to R-134a, I'd replace the compressor and the receiver dryer, and while you have the system evacuated replace all of the seals in the AC system with brand new ones.
Honestly, seals are NOT
expensive, so just replace all of them while the system is discharged.
I had an AC shop convert my wife'* former 1988 Grand Prix from R-12 to R-134a, and insisted they put in all new seals. The converted R-134a system won't blow as cold as R-12, but it'* still cold enough unless you're trying to refrigerate your beer.
97 SE, teflon wiper blades, Dunlop Sport A-2 tires, Leather steering wheel cover, Pioneer DEH-P480MP mp3 CDRW head unit, Pioneer 12-disc changer, iPod hookup, Boston Acoustic 3 way 6x9'*, Boston acoustic separates in door pods, Alpine 4 channel amp in trunk. Magnaflow dual outlet muffler and Ractive stainless tips. Autozone rice pipe and K&N 4750 intake kit. Clear corner lights. Energy Suspension end links.