Cars made before 1995 or thereabouts had the older refrigerants, and millions of them are still on the road today.
R12 is no longer commercially available in the UNITED STATES. It can stil be readily found in Mexico and some other countries.
If you have a refrigerant license, you can still handle R12, and it is available in limited quantities as a recycled refrigerant (junkyards started pumping it out of wrecked cars a few years back).
But for owners of pre-1995 (or thereabouts) cars, the question is, do I convert to R134a, or stick with R12? If you are in the market to BUY a used car, there is a strong motivation to go with a newer R134a car.
I did a "conversion" on my 1985 Mercedes. The kit included a new compressor, expansion valve fittings and O-rings. Initally, many makers said conversions could not be done. Then, they said it could be done, but only by replacing the whole system. Then they backed off to certain components.
Some "kits" sold in auto parts stores claim you can convert refrigerants without replacing the expansion valve and other major components. I am skeptical of something-for-nothing kind of claims.
R134a is less efficient that R12, so it requires a larger evaporator and condensor to provide the same amount of cooling. Since this is not practical in most cases, most conversions live with less efficiency.
The R134a molecule is also smaller than the long-chain R12 molecule. As a result, it leaks though o-rings, shaft seals, and even hose fittings a lot easier. So any conversion has to address these issues, or live with leakage.
(Long chain chloroflourcarbons have long been favored for applications where leakage is an issue. Air will propogate through the interstitial spaces in rubbers and plastics, which is why a certain "air" sneaker actually has chloroflurocarbons in it).
At the same time conversion kits came out, a lot of shady vendors were pushing "witche's brews" of "substitute refrigerants" on the market. Many of these were ill-suited for the systems and could actually cause damage. HVAC "experts" opined that such substitutes could not work.
However, I am hearing stories that some of these "substitutes" may actually work. But I am not sure I would be the first to try.
IMHO, if you are shopping for a car and have a choice, get the R134a car rather than try to mess around with R12 conversions or substitutes. And they are newer cars which will tend to have lower milage.
You can buy R134a all day long at an auto parts store for $8 a can, and you can buy a charging manifold as well. So refrigeration charging on later model cars can be a DIY job, or something done at a local shop without too much expense. Recycled R12, if you can find it, is horribly expensive.
As for the ultimate question, which year the swap was made, I am not certain.
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