I assume you mean an auto repair shop..? If so, did you have to make any modifications to vehicles (like replacing gaskets, etc) to accommodate the restrictions, or was still being able to use 134a sufficient?
As soon as 134a came out we immediately started to experiment. We completely cleared an older system of the R12, we unhooked the lines and blew everything out very well to make sure we had as much oil out as possible and charged it with R134 and R134 oil. Works absolutely great in the older systems and we have been doing it since.
The only thing we ran into over time was the incompatibility of what O-Rings were present in the R12 systems. To prevent these from degrading and leaking over a period of time, we found that all we had to do was just replace all the O-Rings with urethane or silicon O-Rings while we had it apart to clean it out.
So all it costs is some time, labor and the price of a few new O-Rings and you are ready to recharge with 134 and 134 oil. :)