It’s not just the O rings you have to worry about - the spag oil used with R134 will eventually erode the R12 hoses as well as the O rings (new compatible rings are Viton if I remember correctly). Also, R133 systems do not use the POA valve (at least in R12 form), most air conversions recommend gutting the old valve and keeping the shell for original cosmetics.
I guess at some point (when I run out of R12 access), I’ll have to convert my Z11 to R134. Last pricing I saw for a 30lb can was more than I paid for my X77 in ‘73 - no exaggeration.
Regards,
Steve