4 years ago, an A/C hose developped a leak on my old 1978 Corvette , which was on R-12.
My mechanic did not have any R12 in stock so he proposed a switch to R134. I agreed.
He replaced the leaking hose, drained the system completely, installed the inlet valves adaptor and simply put R134 in.
My A/C has run perfectly since then. No compressor problem, still have the same cooling capacity as with the R12. And no expensive conversion neither.
The hose had to be replaced anyway, the valves adaptors cost about $5.00.
True, I did not add too many miles since that repair (maybe 10,000mi), but again, no problem so far.
maybe it can be done that easy on a Merc.