the key of converting to R134 is to remember that the R134 system uses different non compatible oil as compared to R12 therefore don't be like my neighbor and look at seals and o rings and say that they look fine...these components will fail. and you do need a new reciever/dryer. Also keep in mind it's illegal to discharge both systems into the atmosphere.
So change the oil and packings, also the reciever dryer, and most important follow the instructions.
Also my best suggestion is to have it done by a proffessional.
Food for thought - R12 is more efficient than R134. R12 was deemed harmful to the atmosphere only a few weeks before Dupont's patent was due to expire, funny how they had R134 already to go and keep a good corner on the market. Also Halon 1301 fire extinguishant is a VERY close relative of R134.
