The R-12 needs to be done by a facility. WalMart has a conversion kit for about $30 but I do not know if you will need additional cans of charge for the complete system. By '92 need to have the R-12 charge and rumor has it that runs $100. There is a place by I-25 and DTC that came recommended for the job but I can't recall the name now...
--Mike G