I have great respect for Larry and Steve but I must add my free comments (not worth $.02). Here in NC the going price to completely evac and charge a 2 lb system with R-12 is about $200 last time I checked. To do the same with R-134 is about $100. To convert a system with no problems will run more depending on what work needs to be done, for instance how old is the dryer, what switches the system has that need to be changed, etc.
On my personal 84 500SEL, when I needed to change the old A-6 compressor, I converted to R-134. I had problems at first trying to get the correct charge in the system because I had problems finding out exactly how much R-12 freon it was should have. I know, look at the sticker on the rad support, but my car didn't have one. Mitchell says one thing, Alldata says something else, 2 indies in the area told me different amounts which didn't match what Mitchell or Alldata said. And guess what, the dealer came up with a completely different amount.
From this, I believe that the problem with most conversions is not having the correct amount of R-134 in the system. I've explained my proceedure before but in short I add about 16oz of freon. Set RPM to about 1500. Check center duct temp. Let the temp stabalize for a couple of min. Add 2 oz of freon. Check temp again. Keep doing this until the temp doesn't drop anymore. My system has worked great for about 3 years (I think). A friends 87 420 was done the same except nothing was changed except the compressor (used). Converted and charged the same way and he has had no problems and works great.
Now the only thing I can say is logistics and proceedurs come in to play here.
By the way, on a trip to sunny FL. this summer, my car A/C worked fine keeping the inside temp around 72deg. This is in a dark blue car with NO window tent and 4 people.
By the way, a local shop found a great buy on some R-12 this summer and when it was check, he found it contained propane. You should have seen a demo he gave me using some of this R-12 and a match. He was so upset he turned it over to the EPA who told him this is not the first case they have seen.

Sorry for the long post.

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84 500 SEL (307,xxx miles)