I went from R12 to R134a, as recommended by my (now obviously former) mechanic. There are two problems with the conversion:
1. R134a is not as strong as R12, i.e. it will not cool as well as R12 does.
2. R134a operates under higher pressure, meaning a tired compressor will fail more quickly, but worse, you will have a pretty much guaranteed expansion valve failure. Though the part is not expensive, shops charge an arm and a leg to do it. Also, tired hoses will fail more quickly.
I repaired the expansion valve myself, and replaced bad hoses. The compressor lasted three more seasons after the conversion before it became so weak that I swapped it out with a Denso rebuilt that was advertised as being R134a capable.
The A/C is working all right, but not as efficient. I have purchased a parallel condenser that supposedly would add about 10% or more efficiency into the system, but have not yet had the time to have fittings and hoses made to install it. For Washington D.C. thought, with the new compressor, new expansion valve, and so on, the conversion works adequately well.
For replacing the expansion valve, I did a writeup
here. Shop wanted at at least $1,200 to do it, the part is about 40 bucks, and I did it in an afternoon, with the experienced help of my friend Chuck (a moderator of this board).