If you're talking about R-12 vs R-134a, R134a is not an upgrade over R-12. That's a stupidity lazy shops perpetuate to do less work. If you want the best performance from your AC, stick to the refrigerant is was designed for.

R-12 is still available to recharge your system but you have to look for someone who won't insist on switching to R134a. If it came with R134a, anyone can recharge it. Heck, you can do it yourself.

Sixto