I learned AC&R in the Navy and Coast Guard, and before I retired the Coast Guard went on a binge of upgrading all their R-12 units to R-134A. The AC's were fine, but, the refrigeration units were a problem. All the systems used a semi hermetic compressor. Inorder to maintain the same box temps as had been kept with the R-12, the suction pressure would drop into a good vacuum causing air and non condensable gases to build up in the system. I've seen ship A Gangs in arms as to whether they will accept the higher freeze box temp or accept the air needing to be vented off the condensor. The Vigerous and the Reliance had to use the same ship one summer. The Reliance bought a small chest freezer to use for ice cream and prepared to accept the higher freeze box temp. When the Vigorous used the ship for a patrol, they maintained the lower temps, yet had to vent off the air from the condensor from time to time. When they returned the ship to New Hampshire, there was a battle between the engineers on each crew as to why the air was being vented every other watch, and why the box temp was not being maintained by the other crew.
I know that several AC's are sold that use R-134A, and even a couple refrigerators, but, R-134A is not a good replacement for R-12 for use in refrigeration. I would never recommend that anyone simply upgrade their R-12 to a R-134A system, unless it's an AC, like in their car.