I am a bit late for this forum but want to add my 2 cents for new readers. If you convert the refrigerant in an A/C system you must do the following because the OILS ARE NOT COMPATIBLE.!!!! (R134a will turn the mineral oil in a R12 system into grit that will destroy your compressor.) 1. You must replace the expansion valve / orifice tube with a new one. 2. You must replace the dryer with a new one. 3. You must remove the old compressor, drain the old oil out, refill the compressor with correct amount You must then manually rotate the compressor a few times (make sure to have a cup ready to catch the oil) to pump the new oil out thereby flushing the compressor. 4. Install the proper amout of new oil that is appropriate for your refrigerant. (Mineral oil for R12, Ester or PAG oil for R134a) look up your compressor for how much oil is need for that specific compressor. (Do research about how much oil is needed for your vehicle.) 5. You must FLUSH out the Evaperator, Condenser, ALL hoses with a good flush solvent. Then blow them out with compressed air. then repeat. (Catch the solvent and examine for particles, if particles continue to appear repeat until clean.) These particles can clog you compressor and cause it to fail. 6. If you switch from R12 to R134a you should consider converting to a parellel flow condenser which has much more volume then a tube and fin or piccilo type condenser. Warning: You cannot flush a parallel flow, the holes are too small so if it gets clogged in the future you must replace it. I have read a lot about this and it is suggested that you install and inline filter to protect your compressor for the future.
There are many more details so I suggest you reseach auto a/c forums for help on the specifics.