We have been doing retrofits from r12 to r134a for years now. It consists of, drain the r12, drain as much oil as possible, measure it, put in the equivelant amount of new oil, red loctite on fitting adapters and fill. Never had any problems, the odd expansion valve freezing, thats it, and still very rare. There is 2 types of oil, ester and pag, i think, i beleive in a brand new system, pag is better, but in a retrofit the ester mixes better. I don know some Ford and GM cars have the odd issue but for the most part, converting is fine