Chris,
My '91 Carat has been converted to R134a.  However, this was in conjunction with a leak diagnosis.  What happened to me was this:  my R12 system leaked out in a matter of weeks.  Living in FL, driving with A/C is definitely preferred.  So, off to the mechanic I went...  System charged and holding, very odd.  But, start the engine, engage compressor and viola! leak found!  The compressor was at fault.
So, new compressor and all is well.  All that was changed, in addition to the compressor, was the receiver/dryer (Bus Depot).  Other than that, the system was flushed, and a good vacuum was pulled on the entire system.  My van spent the week in the shop to make sure that the fault was found.  Thus far, running R134a it is GREAT!  I was considering the front A/C, but this thing is so cold (to date) that I don't need it.  And, remember I'm in FL, it was already 81F this morning around 7:30 -- so I use the A/C daily now!  In direct sunlight (I don't have tinted windows yet either!) my system still manages to keep things cool.
Now, as for o-rings and such, I don't know for sure if my mechanic changed them.  But, since the entire system was disconnected and checked (during diagnosis of the leak), I'm thinking that any such consummable items would be replaced.....  still, not really sure.
Good luck!
    -Robert
     '91 Carat, 149K miles, original engine

-----Original Message-----
From: 80 Westy Pokey <pokey@VANAGON.ORG>
To: vanagon@GERRY.VANAGON.COM
Sent: Wed, 16 Jun 2004 10:56:51 -0400
Subject: R12 to R134a Air Conditioning Conversion

Anyone done one of these? What does it entail?

Thanks,
Chris