$40 for a can of Freon? I think /not/.

Converting to R-134A is easy to do but there are some things you must know when doing it. First lets put away some of the myths. R-134A doesn't leak any faster than R-12 did. Remember you must have had a leak in the system to start with otherwise you wouldn't need to charge it, R-12 or not. So whatever you put into it would leak out unless you repair the leaks first.
Sencondly & more importantly is the older R-12 systems were designed for R-12 which will run at lower pressures in the same ambient air temperatures than R-134A does. So when retrofitting a system to R-134A you should ONLY ADD ABOUT 80% OF THE SYSTEMS R-12 CAPACITY WITH THE R-134A. So if a R-12 system needs 2lbs of charge you would only add 1.6lbs of R-134A. That will keep the pressures where they need to be & you won't lose cooling efficiency due to system overcharge. It is compatable with the R-12 type oil in the system already. Changing the desicant in it isn't absolutely necessary unless you have opened the system up for repairs, but it wouldn't hurt if you so desire.
Lastly, I would recommend against using ANY of the so called replacements out there. They don't really work better & in order to be compliant you need to have a different service adapeter set installed on the car & it should never be recovered into a machine that is used for any other type of refrigerant. EVEN if it's what it claims to repalce. That's federal law! You must have a dedicated machine for each type of refrigerant used & they must use differnt fittings so to avoid contamination. Besides, R-134a is so much more readily available & isn't that expensive anyway.