In addition to the components required to retrofit an A/C system to use R-134a, it should be noted that as R-134a has a different pressure/temperature curve than R-12 there will be more pressure at a given temperature in the high pressure side of the system, often requiring a larger and more efficient condenser, or at the very least increased airflow over the condenser core to dissipate the heat generated. Equate this to the radiator in your cooling system and you'll see why an electric fan and shroud is important to help boost the performance of the A/C system.
The Future
Though in Europe R-134a will start to be phased out by 2010 in new cars, and disappear completely by 2017 (as it is a greenhouse gas that contributes to global warming when released into the atmosphere, even though it doesn't harm the ozone layer) American and Japanese automakers will continue to use it, though A/C systems will be redesigned to use less refrigerant, reduce leakage and improve efficiency. However, at some point R-134a will likely be replaced worldwide. CO2 (R-744 when used as a refrigerant) and HFC-152a are possible alternatives, along with HFO-1234yf. A refrigerant developed by DuPont and Honeywell, the latter meets the new European Union regulations which will require a refrigerant to have a global warming potential (GWP) of less than 150. HFO-1234yf has a GWP of 4 compared to 1430 for R-134a and looks to be the safest and easiest replacement, especially as there are no major alterations required to use it in an R-134a system.
Old Air Products' POA Update...
Old Air Products' POA Update Kit replaces the original POA valve and uses a pressure switch to convert it to a cycling clutch system as found on later model vehicles. Compatible with all EPA-approved refrigerants, this kit is ideal for use in an R-134a retrofit situation.
Typically found on '62-65...
Typically found on '62-65 GM vehicles, the STV (suction throttling valve) is essentially a regulator, located at the evaporator outlet and controlling the amount of compressor suction and therefore refrigerant flow through the evaporator.
Old Air Products STV Update...
Old Air Products STV Update Kit removes the original piston and diaphragm and converts the system to a cycling clutch system. As with the POA Update kit, this is also compatible with all EPA-approved refrigerants.
VIR valves can be found on...
VIR valves can be found on GM vehicles from '74-76, admittedly a little late-model for most of us, but they also appeared on other makes using GM-designed A/C systems. Mounted at the outlet of the compressor, the VIR includes a thermostatic expansion valve, POA suction throttling valve and the receiver/drier.