As long as R-12 is available, there is no real reason to convert older vehicles to R-134a. That is because R-12 systems cool best when charged with R-12 refrigerant. But converting to R-134a does make sense if your A/C system requires major A/C repairs (such as a new compressor, condenser or evaporator). The extra cost involved to make the changeover to R-134a does not add that much to the total repair bill.
A basic retrofit procedure can be done one of two ways. The "Type 1" retrofit follows the OEM recommended procedure and generally involves removing all the old mineral oil from the system, replacing the accumulator or receiver-drier with one that contains a desiccant (X-7) which is compatible with R-134a, replacing O-rings (if required), installing or replacing a high pressure cutout switch and/or orifice tube (if required), then adding the specified PAG oil and recharging the system with R-134a. Federal law also requires the installation of R-134a fittings on the high and low service ports to reduce the chance of refrigerant cross-contamination the next time the vehicle is serviced, and labels that identify the system has been converted to R-134a. For more information, see the R134A retrofit guide program featured on this web site.
A "Type 2" procedure is more of a "quick and cheap" approach to retrofit. On many 1989 through 1993 vintage vehicles, the R-12 A/C systems can be converted to R-134a by simply recovering any of the R-12 that is left in the system (NOTE: it is illegal to vent refrigerant into the atmosphere!), adding POE oil (which is compatible with both types of refrigerant), and then recharging with R-134a.
NOTE: Type 2 conversions cannot always be done on some vehicles because their compressors may not be compatible with R-134a (any compressor with Viton seals has to be replaced). This includes original equipment compressors such as Tecumseh HR980, some Keihin compressors and some Panasonic rotary valve style compressors in older Nissan vehicles.
Durability is another concern. Because R-134a raises compressor discharge pressures and increases the compressors work load, some lightweight compressors may not be rugged enough to tolerate R-134a over the long haul. This applies to the Harrison DA6 and Ford FX-15 compressors. The Harrison DA6 can be replaced with a HD-6, HR-6 or HR-6HE compressor. The Ford FX-15 compressor can be replaced with a FS-10 compressor.