I would not retrofit to R134a, unless you have leaks or other issues that need repair. IMHO, R-12 is much better at cooling than R134a. However, if you need to, or decide to convert, go to
www.ackits.com
They have their own open forum and many mobile a/c techs answer your questions/posts accurately, and usually fast response from someone online. I am not advertising them by any means, I have gotten alot of useful info from them in the past and the website has any type of a/c part for vehicles that you might need. I have never purchased anything from them, yet.