Not blowing cold air

R12 is not illegal, however I believe it is out of production. You can still buy it but its expensive and you have to have the proper license to purchase.
I recently had to get a license just to buy a 30lb R134 so the laws are steadily changing.
Yes you must replace the dryer when converting from 12 to 134. The proper thing to do is replace it any time the system is opened. Also, R12 and R134 dryers use different dessicants in them. Pag oil and Ester oils should not be mixed so the system needs to be flushed.
Not much room for error when servicing and AC system. Sounds like you need to hire someone.