Well, FWIW, I converted my '85 Westy to R134-A in Sept. of '02.

What I did was, I replaced the receiver/dryer, removed the compressor
and drained the old oil out and flushed it, flushed the system, replaced
only the O-rings that were disturbed, put Ester oil in the compressor
and put it back on, drew a vacuum in the system and let it sit for about
3 hours then added oil and refrigerant.  I didn't replace the expansion
valve.

I've had to add a little refrigerant each spring but the charge
certainly doesn't leak right out.  My guess is that the old mineral oil
coated the old style hoses over the years which made them less prone to
leakage with the R134-A.

Mike