1995 Chevy Cheyenne My air conditioner needs to be re-charged

1995 Chevy Cheyenne mileage: 103,800. My air conditioner needs to be re-charged. It has the old type refrigerant so it needs to have the new type put in. Problem is my local garage said the manufacturer recommends not doing this. But their computer tech manual gave no reason why. Why does GM recommend not doing this? If I have it done, will it cause problems with the AC unit? If I can't have the new style refrigerant put in where would I find the old type? (I don't think it's made anymore)