R134 has a smaller molecule than R12. Supposedly, you should upgrade the hoses so it won't leak (very slowly) through the hoses. Be sure to get out as much of the R12 oil as you can. Also, replace the drier, It will have moisture and R12 oil in it. I know, said already! There are aftermarket filters you can put in-line if you're worried about any junk floating around the system.