My compressor died in mine a few years back and I realized at that point, in the middle of August in Florida....lol, that a/c is not optional. I decided to convert mine over and thought I would pass along my experiences. Just my .02
The problem with just dropping 134 into a r-12 system is an oil circulation/missibility issue. As long as you disconnect all the lines, flush the evap, condenser and lines with a system flush such as RX-11 or something comperable to flush out all the mineral oil from the system componants. I used brake cleaner backed up with dry nitrogen to flush mine when I converted and I also was replacing the compressor so the new unit was dry but if you are using the old compressor it needs to be drained of all its old mineral oil also. Reason this is important is because mineral oil and POE doesnt readily mix together and will pose a problem. A small amount can be left behind and it will be fine. The evaporator especially needs to be flushed well because if a system had a leak and was running with a low charge, the velocity of the low side vapor drops, the oil will log up in the lower bends of the evap coil and be left there. Now you install a new compressor or add the proper charge of POE oil and start the system up fully charged with 134, all the old mineral oil gets flushed out of the evaporator and into the compressor which results in either an overcharge of oil, an incompatable mix of mineral/POE oil or both which either way isnt good.
Also, when doing the conversion, its a good idea to change the accumulator/drier and suction line that goes with it back to the compressor, the liquid line from the cond. to the evap. which includes the orifice tube and upgrade all the snap-lock fittings with the better quality 134 o-rings. Always coat the new o-rings with some POE oil before installing them too. Another important item is ensuring that a proper vacuum is pulled on the system after all the componants are reinstalled. POE oil, unlike mineral oil absorbs moisture very readily so removing the moisture from the system before charging is imperitive. Also, when working with the oil, keeping the container closed and the cap on tight is equally important also. Before starting the system, the low pressure cycling switch that sits on top of the accumulator needs to be adjusted down about 5 psi or so so that it will cycle the system at the proper temp. As it is set for 12, if left alone it will cycle the comp. too soon and not let it get cold enough as 35psi for 134 = 40* and 35psi for 12 = 36*. Set for about 30psi seems to work well.
Another note, before I did my conversion, I had also converted to a taurus elect. fan and that seems to help with the complaint some folks have that the air temps coming from the vents rises a bit sitting at idle like in traffic or at a light but cruising down the road it blows nice and cold. Having a clutchfan will make this effect more noticable. I understand that changing out the condenser coil to one that is 134 specific helps this alot.