I am going thru this right now on my 89 XR-7,
This is what I have done, change the drier, change the condenser and the liquid line.Now you need to make sure your system is emptied of the mineral oil from the R12. What you should do is have the system checked for leaks and flushed.Once you know all is well have it filled with 134 and make sure they put the compatible 134 oil in it.Its very important not to mix the oils,you will need to put the 134 quick connect fittings on it as well( Red and Blue caps), these fit over the existing high and low side servicing ports.
Also make sure you change the o-rings, spring locks and the most important thing to change is the accumulator/drier
Walmart has the cheapest R134a, I have ever sceen, 9.99 can. *bay R12 $20 can plus shipping, typical. Most cars require you to open 3 cans.
I converted an my 85 Camaro to R134a. Installed new dryer, oil, orifrice and reman compressor. Didn't cool nearly as well as my 91 firebird, w/ R12. Same basic AC system.
A redesigned condenser makes a big difference. I've tossed out at least 2 older tube&fin style OE R12 condensers and used either a 94+ model or aftermarket replacement. The newer ones have 6mm tubes with 2-3x more turns and shed a lot more heat. As garsc has said, the biggest factor in a retrofit is the condenser. I've found that to be true as well. Bear in mind, I live in a high heat state, so while some people further north have good results with the original condenser, its just too darn hot here.Walmart I converted an my 85 Camaro to R134a. Installed new dryer, oil, orifrice and reman compressor. Didn't cool nearly as well as my 91 firebird, w/ R12. Same basic AC system.