The compressor will need to be removed to pour the old oil out and the receiver drier should be replaced, R134 PAG oil is not compatible with the R12 oil. I don't remember if our systems have an orifice tube or an expansion valve but that should be removed to be able to flush the system, if it's an orifice tube replace it. Also, the system should be charged with dye added so you can tell where any leaks are after the system is charged. If you don't use dye the refrigerant is invisible and most likely won't be able to find a leak. Of course, the R12 service ports will need to be replaced with R134 ports. For reference, if we did it at our shop, and your compressor was good, the cost would be roughly $380.