The story you always hear about expensive R-12 to 134a conversions is not necessary. When I went for my refrigerant license last year I asked the instructor about this since my olds is R-12 and needed a charge. The problem is the oil in the refrigerant, 134a uses a different oil than the 12 and when mixed it turns into cottage cheese. However if you just have the shop completely evacuate the system you can charge it yourself with 134a and should be fine. Be sure to use the high mileage freon with the seal conditioner. I've heard several stories about how you have to replace this and that but nobody has given me any proof. I charged my Olds a year ago and ran all summer with zero problems, Ice cold. Bought the kit with the adapter fittings at Wal-Mart in Chicago for 35.00 and came with everything including 3lbs of 134a and instructions on a CD-ROM. I had it evacuated at a buddy's shop for free, but it shouldn't be much to have it done. Also anytime you remove a line from an AC system always replace the seals, they're cheaper than refrigerant.
I hate DuPont. All this changeover crap is because their patent was up. DuPont held the patent on R12. They conducted the study on the Ozone damage that they say R-12 caused. All theory, nothing proven and all done by DuPonts lab. Then they lobbied Washington the have it outlawed because they got the patent on it's replacement the 134a. Now their monopoly holds. How convenient.