Although we convert cars every week and haves done hundreds of them, I personally give the advice to leave it R12.
It confuses me the talk of having anything else be more efficient (money wise). R12 at $60 a pound is still the cheapest component (at list price) of almost all pieces of the system. The way people talk its as is this was a constant problem. Since I can't imagine someone just adding refrigerant to a leaking system, I have this problem understanding the problem.
I approach every A/C job with the concept that this is the last time it will ever be worked on. A/C units work for years. Repaired ones should do the same. The fact that R12 may be totally unavailable someday will not make that repaired A/C work any less. Today, with current availability of R12, the price of the gas charge has risen to the bottom of the component list. Freon appears to have stabilized in price and I have no problem buying it (except capital outlay).
From the experience of all these conversions I can give a few conclusions after ten years. Under most conditions the use of R134a will cause an old compressor to leak in as little as a year and probably 50% will leak by three years. New compressors will last 50-75% of previous intented life. This means some failures within a couple years and significant ones after 3-4 years. I attribute this to heat, pressure, and small efficiency loss which causes more on time.
Cars A/Cs deminish in efficiency over time. Coating occur on both heat exchangers inside and out. Unless a system is kick-axx I avoid conversion. I have had many, many people tell me their A/C was never as good before I converted them. I always believe the case is that they don't remember when their A/C performed properly and the 5% lack of efficiency is inconsequential to this condition. A mediocre A/C that is being worked on for a leak, shouldn't be converted. Fixing the leak won't make the system any better, just full. Putting 134 in will probably be noticed.
Bosch Master, ASE Master, L1
33 years MB technician