There are folks out there that can do the modifications. Usually they leave the A2 sight in place and put a weaver type rail forward. I believe that was more commonly done when A3 type recievers were not the norm. With the cost of a new upper reciever being approximately $100, there really isn't a reason to try to convert your existing upper. The cost of the modifications would easily equal the cost of a new upper reciever, at the same time leaving your old upper valued even less then original.
Good SHooting
RED