R-134 vs R-12 and moisture too
I've had the experience of having my mertz converted and the techs then converted back and told me I just wouldn't have been happy with the performance from the R-134. The reason they gave me for lackluster performance for the R-134 conversion was that the (over)design of the system was such that the heaver components didn't cool as rapidly. With 100+ heat in TX we want all the cool we can get + as quickly as possible. The experts don't recommend conversion if quick cooling is wanted (although they told me that eventually the R-134 would have also cooled the system sufficiently (probably about by the time I was turning into my garage). The R-12 keeps the car instantly cool even with the Rag-top on.
Second topic: getting all the moisture out of the system is essential to keep the water from reacting with the freon to form acids that will destroy your system (eventually). Definitely change the drier, and flush the system. R-134 conversion also requires a new expansion valve for the R-134.