Whether or not a player is left or right handed can be easily changed in players.dbf. Just open it with DB Commander, locate Reggie Williams, and change his HAND value to 1.
As far as his shooting form, that can be changed by editing the SIGSHOT field but we're limited to what's in the game already. If there's a signature form that suits him, his SIGSHOT value can be changed accordingly (ie look up the value of a player whose form seems suitable and give that value to Reggie Williams).