Hiyou can't mix refrigerants, and the system will be way less efficient on R12 as it has different characteristics to R134. Check oil compatibility as well, some of the R134 oils are not compatible with R12a
If you want to convert it to R12 and accept the 30% approximate loss in cooling efficiency on this vehicle given the evaporator and compressor sizes then get it done professionally, they will change out the valves to fit the R12 gas connections, flush, fit a new filter / dryer and vacuum the system to remove all the old gas, then refill.
Do not mix the 2 gases. Compressor damage will be the least of your worries.