The kits on the aftermarket are for chanageover and do include oil and quick-disconnect valves so that one does not use an R-12 manifold and contaminent the system.
Some I know have had good luck and some bad. If one were to change, I'd recommend changing the receiver-dryer, all O-rings, and a complete flush of the system before a total evacuation.
Compresssor pressures will be higher, i.e., R-12 = 275*, R-134a = 300*. Cooling will not be as good, maybe at best, 80-85% of R-12.
However, it will work good if the system does work good now.
Maybe some others will chime in here and offer their opinion......