Changing the system from R12 to 134a is not a task you can do very easily with tools at home and can't be done without the right equipment.

First the whole system needs to be evacuated of the R12( if any), which needs to be disposed of safely or recycled. Then all the mineral lubricant needs to be drained out with a system flush. The accumulator and/or dryer has to be changed to be 134a compatible, not to mention all the old o-rings, pressure valves and the orifice tube.

Then if the system holds vacuum, A/C lubricant has to be added and the system evacuated again. Then the system gets charged with the 134a, adjusting the pressures will adjust the compressor cycling by reducing the pressure for the cutoff switches for best cold temperature.

 

I doubt there's any freon left in it. Yes you have covered the required recover and recycle, we all know everybody does that. . I never had a problem just changing the orifice tube, vacuum and recharge, except compressors like to die after long term non use.

 

All you need to do to convert is pick up a conversion kit. It adapts the connections and comes with new oil. The included gauge is junk but you can get the job done. It's a good idea to use a system flush first to remove the old oil. The new synthetic works fine with your old equipment assuming its in good shape. You will want a vacuum pump to evacuate the system and yeah it should be recycled properly. I did mine 3 years ago and its been ice cold since.

 

Don't run the compressor for more than a few seconds without lubrication in it. If its not already dead that will kill it fast.