From what I've read and done, you can buy oil that doesn't react badly with the R12 oil - a 'conversion' oil that will help the R134 bond to the R12 oil. When I did my system, I had it totally evacuated, along with some manual draining that I did - which still leaves something in the system.
At any rate, if you're going to drain the compressor, you'll need to pull the condensor out of the truck and drain it as well.
Thing is, the conversion kits come with a set amount of oil to freon mix, and the oil works with the R12 oil. I'd suggest doing something along those lines first, before doing a total system flush/drain etc.
One consideration to be concerned about on the flush - if you're using compressed air, there could be a problem in pushing air and oil through your expansion valve. I'm not sure how it works, but when I thought I'd over-oiled my system, my AC guy warned be about using the compressed air to flush the condensor, and suggested that I pull the condensor and let it drain on a bench.
I'm just parroting what I was told, actually. I'm not exactly sure how an expansion valve works, or how sensitive it actually is.
I do think it's a goo dmove to drain out all of the oil and start from fresh though.
Bench drained the compressor. Compressed air to push flushing fluid through the condensor and evaporator. Refill with 134A. Ambiant temp of about 90, cooling to 50 at the vent. Pretty good, IYAM. We are going to run some stop leak in the system for a couple of weeks then drain, replace dryer and refill. This is on Jester's truck. Mine hasn't gotten the full drain procedure yet. On mine I added oil and refilled and have about the same temps, but the site glass is cloudy on mine and clear on Jester's.