I agree with Larry. Get yourself a 609 certification and stay with R-12.
It's my belief that systems that come from the factory with R134a work fine for the most part. Some retrofits work; many do not. In order for a retro to work and I mean work for the long term, a long list of tasks must be performed correctly. Even when this takes place, people are often disappointed in the results.
Steve Brotherton had an interesting comment about R-12 not so long ago. It's the cheapest component in the system, yet people are in awe that they have to spend a few bucks to keep cooling with it.
If your system is leaking, find the leak and fix it. You can convert to R-134a, but you're still going to leak.
My 2 cents.