Here is a web page to explain the general procedure.
http://members.tripod.com/~jbabs714/autoac.htm#134
There are several opinions as to what has to be changed.. since R12 and R134 systems require different compressor oil. Some say that the entire system has to be flushed completely which requires a lot of work and perhaps a change of dryers (dessicant). Others say that you dont have to do all this stuff now....however, uncle sugar has specific rules that should be followed. The R12 is not supposed to be blown off to the atmosphere anymore....it has to be "recovered". A lot of shops will gladly "recover" it for you (remove it) since the stuff is worth $50 a pound or so. Also, there are specifics regarding the port change (charging ports) and the labeling of the system (must have a retrofit label). Do some research... the $1100 might include a new dryer, new compressor, etc while a cheapie job is just changing the gas and the ports.

Why do you want to change? R12 is a better referigerant and provides better cooling....only slightly better. It is quite a bit more expensive...your system probably holds somewhere in the 3-4 lb range unless you have rear air (adds a small bit)

mike in tucson