The instructions on the kit say to completely evacuate the old system, but that would require a vacuum pump which I don't want to buy just for one job. I know it isn't legal to vent the R-12 into the atmosphere, but I think it's done quite a bit anyway.
Those of us with redneck friends have been converting like this for years. Southern rednecks have more yankee ingenuity than most folks.
I am a little nervous after reading an earlier post regarding the new stuff attacking the hoses.