Do you have to remove all the R-12 refrigerant before upgrading to R-134a?

Yes, it is necessary to remove all R-12 refrigerant before upgrading to R-134a. The two refrigerants are not compatible, and mixing them can lead to system damage and reduced efficiency. Additionally, proper recovery and disposal of R-12 is required to comply with environmental regulations. After removing R-12, the system should be thoroughly flushed and modified to accommodate R-134a.