Hi I am an air con and refrigeration mechanic and thought I could offer a bit of help, you can change from R12 to 134a refrigerant with minimal hassles just need to remove the oil from the system by flushing it out until it is all clean then it's just a matter of having a new drier/filter installed, the new hose installed and then having the system vacuum tested over no shorter than 4 hours to remove any traces of water/condensation. Once all is ok it is time to recharge the system with the new 134a gas and compatible oil for your system, should only take 5.5hrs to do the job professionally not hard to do just a matter of checking pressures while testing to make sure the system is charged to the right level, too much gas and it will flood back to the compressor and hydraulic not enough gas and the liquid line from the condensor to the evaporator/TX valve will not fill with liquid refrigerant/oil mix and you won't be getting the most out of the system.
Never believe someone if they say "the air con just needs a regas!" air con systems never need a regas they just leak and then require the leak to be repaired and then regassed it's like most people think that the gas goes bad or something hope thats of some help any other questions just ask