My first order of business would be to have the mechanic confirm that the oil is R12/134 compatiable. It most likely is compatiable.
Then find out if the desiccant in the R/D is R12 compatiable. It most likely is.
Lastly, find out if the expansion valve was changed out and why. I haven't heard of many R134 specific expansion valves for conversion purposes. If it was changed out it was probably done with the seal changes (if it uses seals vice copper seals) since it had to come off any way and they aren't normally very expensive.
Since the system is probably not leaking, it would then be a matter of a pull down, and recharge with R12.
Compressors aren't cheap, and one working too hard will fail sooner. Benzmac hit the nail on the head for me. R12 is the least expensive component in the A/C chain. Why mess with a good thing.
S, J.R. Brown
2000 G500 LWB Obsedian Black
2005 Toyota Tacoma Access Cab Off Road Sport
1993 Volvo 240 Sedan Anthracite
1980 450SEL Champange (owned it for 15 years. Great car)
1986 280GE LWB Anthracite (Sold it and kinda wish I hadn't)