Hmm, I thought you have to replace the oil since the oil they use for R12 is not compatible with R134a and will deteriorate. R134a isn't as stable as R12 is.
You may also want to consider switching to DuraCool as an option. This is a HC based refrigerant instead of an HFC based refrigerant . It should be as simple as charging the system. It should also cool better than both R12 and R134a.
http://www.duracool.com/
I haven't tried it yet, but I think I'm going to.