yes it is a good idea to convert to R-134a because R-12 can be as much as $200 per lb.
however jeep freak in incorrect by saying that all you need in the fittings.
the accumulator (filter) will need to be changed, as well as getting as much of the old compressor oil out as possible. the reason being that R-12 uses a mineral based oil, and R-134a uses a synthetic PAG oil. The mineral oil will not "disolve" into the R-134a
and thereforer not get carried through the ac system. Leaving the ac compressor with a lack of lubrication and will most likely seize. also since the mineral oil cant be absorbed by 134a it will just sit in the bottom of the ac system taking up room that would otherwise be used for refridgerent, and the system will not blow as cold as it could.