I don't understand the logic here. The guy converted it to spring, said it would have cost the same to fix the air ride, and now you want to do it all over again?
Don't.
If you want air ride, find one with air ride.
Or, take comfort knowing that you're not going to have to walk out to your car one afternoon, at the worst possible time, and find that it's lying on it's belly, or the back or front end isn't extended to the fullest while the opposite side sits inches off the ground.
If any one component are missing, broken, or fried, the cost and ease of the conversion will increase exponentially. Those little plastic hoses that run through out the car are a pain in the rear to trace and fix/replace.
If you bought the car already, just enjoy the peace of mind.