Being a GM tech for 16 years I would retrofit to 134A. I have done a lot of retrofits with now problems at all and I don't where most people are located but here in PA R12 is very expensive and hard to get. It usually pays to retrofit then to add the R12 again especialy if you have repeat freon leaks. The cars that I have done including my own(92 Elderado) had the same vent temp or colder. If done properly it works very well but some people just evacuate the R12 and add 134 which is not a good idea. Here's what I do. I evacuate the R12 twice, on most cars I replace the acumulator where the desicant bag is located, change the fittings, connect the ACR2000 134 machine and flush the system. The ACR2000 actualy flushes the system and all of the oil with 134 for about 15 minutes, on most car you have to remove the orifice and/or expansion valve when flushing. It's a good idea to replace them anyway if contaminated with metal or desicant. Then I evacuate and add the correct amount of PAG oil that is recommended for R12 and 134 compressors. Then I charge system. You have to add a little more 134 then the recommended amount of R12. I think it's like 10% more but I just add it until it gets as cold as possible. Well worth it!