Sanden AC compressor units are designed for specific types of refrigerant, swapping over to R134a will require the change of all internal parts of the system. It is not recommended that compressors designed for R12 are used with an R134a system.  To do the change from R12 to R134a cycle the system and then capture all the R12 with proper equipment. Be sure to drain all of the mineral oil out of the entire system before beginning the retrofit.