If your going to go through with it then it is essential that you have a professional do it. Just about every component will have to be replaced with the exception of the condensor and the lines which will need a thorough cleaning as the compressor oil for 134 and R12 are NOT compatible. For more info see this link.
http://www.aa1car.com/library/retrofit.htm
I think its pretty accurate. Im not an A/C mechanic so I don't know all the particulars but read the article as it makes sense.
mrnickleye said his shop does the conversion for 210 dollars, thats probably the labor cost without parts which seems very reasonable but can vary slightly depending on the vehicle. It seems whenever I ask about the swap Im usually told 500 dollars from places like pep boys and other chain stores,
and also just for fixing a leaky system.
I got my Blazer done there before and the total cost was over 800 dollars. That was changing the old compressor with a rebuilt one changing the receiver/dryer and charging the system. The only good thing that came out of it was the mechanic left his cheap flashlight in the engine bay, and my system worked for about 3 months till I went back to Arizona and found out that their Rebuilt compressor was leaking.
One other thing, the article mentioned an oil that is compatible to both R12 and R134 but the old oil will still have to be thoroughly cleaned and any old refridgerent properly evacuated.
Let us know what you do and how it works out.