[many944s]

+1
Both Duracool and Freeze 12 can be added to an existing R-12 system with no changes. If your system is "opened" for any reason (changing compressor, dryer, o-rings) you should pump it down to a vacuum before adding your refrigerant. If the system still has pressure, you can simply add the new refrig.

Why to evacuate system:
Pumping the system down to vacuum for half an hour or so pulls the majority of the "condensible" materials from the system (mostly water vapor from air). If there is air or moisture in the lines, the A/C wont function as well.

I would advise staying away from the R-134a conversion kits. For the system to function well over the lifetime of the car, you should drain ALL of the original Mineral oil from the compressor, change the seals and o-rings, swap out the dryer, and use a good clean PAG oil charge, it costs a little more than the $39.95 box on the shelf that says "R-134 conversion kit, everything you need"

Hope it helps!
-Nick