Going from R12 to R134 is not the problem that everyone thought it would be 10 yrs ago. R134 runs at a slightly higher pressure than R12 but not enough that you have to replace the expansion valve or expansion tube. R134 is a blended refrigerant so therefore you only use 80% of what your R12 calls for. The biggest problem is the incompatability with certain types of compressor oils. Flush you system with an A/C flush, change your compressor oil, add the R134 and be on your way.