I generally recommend not converting to R-134A. R-12 can still be had and works better.

First, I'm sure you know that you have to flush the entire system very well, as the oils used for the different refrigerents are very incompatible. Flushing through the expansion valve and getting everything out of the condensor can be next to impossible without removing them completely first.

Second, R-134A requires replacing all of the o-rings with green ones that are compatible. The black ones used for R-12 will get eaten away.

Third, sometimes you have to replace the hose set with barrier hose, as R-134A is a much smaller molecule, and thus can diffuse through some materials easily.

Fourth, your compressor will not be as well lubricated with the PAG oil used in R-134A. The oils used with R-134A are not as misible in the refrigerent (doesn't dissolve into the flow of the refrigerent and carry through the system as well), and thus the compressor suffers, eventually dying the 'black death' and having black crap go through your entire system, necessitating the replacement of the condensor, evaporator, expansion valve and dryer.

As you can tell, I am not a fan of conversions. I've done it many times (I'm a mechanical engineer and I have an AC cert), and have never had a conversion last more than a couple years before problems popped up. (Three years on average, and the problems usually started with compressor noise.)

On my mercedes, I actually converted it back after a year to R-12 after removing and flushing every single line and component and replacing the dryer (you are going to use a new dryer right?). The R-134A never worked right with the climate control computer on that car.

Anyway, that's my experience with conversions. Some people have had no problems....

Good luck