I took mine to an a/c guy, told him I was removing the system and needed to evacuate the old R-12, He said to reclaim the R-12 he would purge the system and run pressure to it to make sure it was completely purged and empty. Cost to do this???? Free of charge, as he got to keep the reclaimed refrigerant. I then went to advace and for $35 got the 134 conversion kit. and followed the directions...... Results. Cold blowing A/c in my XJ, that I can check and refill as neccessary. No special tools, no a/c tech charging an ungodly amount because they can. Simply remember this, the r-12 system WILL NOT work the same as it did before when you add the new refrigerant, and that the two work similarly, but not exactly the same, therefore the tolerances and pressures are less forgiving with the new R134. However, as long as you have no leaks, or repair them, and stay within the recommended charges you should be satisfied with your a/c system.
If not, there is always the option of 4/55 a/c and onboard air!!