Cant be done b/c the gas block is in the front sight assy on the shorty, and theres nowhere to mount the front sight assembly on the front of a "shorty" barrel. ummmmm, something like that. You would need a new barrel, new gas block, new front sight assy, new gas tube and new handguards, and someone to put it all together on your upper receiver.
Anyway, do what I did, list your upper in the equipment exchange forum under wanted to trade. It took 2 days for me to find someone to trade my shorty upper for a dissy. Thats the easiest, cheapest conversion ever. ($8 bucks shipping is all it cost me)