That sounds like a ripoff - you can flush the system, change the drier and expansion valve, and swap out the O-rings yourself. Use ester oil instead of Pag oil for the 134A. The expansion valve doesn't necessarily need changed out, but swapping it out will improve the efficiency, because the orifice in the r12 expansion valve will cause less heat removal due to the larger size of the orifice. The compressor doesn't care which refrigerant it uses, once the oil is changed. With the ester oil, you could leave the mineral oil for R12 in the system - it will not react with it. Pag oil, on the other hand, will react violently, destroying the whole system.