I've done this a couple times...
Bear in mind that R134 apparently has higher working pressures, thus the difference between R134a and R12 compressors. You're likely to be changing a compressor soon after the refit.
The receiver-drier/accumulator (whatever that thing's called - under the compressor on the low-pressure side, with the LPCO switch on it) will need changing - after you evacuate and flush the system. The accumulator has a dessiccant (sp?) in it, and has a habit of hanging onto the R12 lube. Mix the Ester (R-12) and "PAG" (R134a) lubes, and it's like foamed glue - wrecks everyting.
There is enough differences between the two that changing the hoses isn't a bad idea, either. They're probably due anyhow.
Having said all that, there's a "hydrocarbon" refrigerant out there, instead of a halogenated hydrocarbon - it's supposed to work better than the R134a or the R12, and be easier on parts. Chcek it out here -
http://www.autocool-refrigerants.com/
I'm going to get hold of them and get some more information, but all you apparently need to do is evacuate and flush, change the service valves to R134a-style, and fill with Autocool. Compatible with pretty much any oil, and lower head pressures than R12 or R134a - so it's easier on compressors.
5-90
