I'd like to amend my earlier comments, along w/ any necessary apologies for swaying folks one way or the other. I've been doing a bit more web research and came across the following links:
Automotive Air Conditioning Conversion to R134a
R-12 retrofit
Convert your automotive air conditioner from R-12 to R-134 Refrigerant.
AC Refregerant R12 to R-134A?
These pages suggest that a lot of what I've been told by AC shops may be more driven by profit than truth. Perhaps the conversion is much simplier than I was led to believe.
I suggest doing your own search (I used Google) and seeing what comes up. There is definitely some confusion out there. Earlier pages talk about changing compressors, driers, lines, o-rings, seals, etc. Later pages talk only changing driers. More recent pages talk mostly about making sure you have a good shop do the evacuation of the old R-12 (and possibly lubricant oil) and then you should be good to go w/ the relatively cheap ($25-100) conversion kits that appear quite easy to DIY.
Good luck and keep us all posted. I know many folks will benefit from whatever you find out.