My 1986 PORSCHE has R12 can I add 134a into the system or do I need to vacuate? on 1990 Porsche 944

I have guages and am mechanical inclined, do I need to evacuate the system or can I add the 134a with the R12?

Asked by for the 1990 Porsche 944
Need to change ovet to R134A per service manual. Don't mix them.