Pallet Rotation
I have a 1996 HAAS HS-1 and there are several things to do before the pallet will rotate. First, the code to do a pallet change is M50, but on newer models you might have to specify P1 or P2 (not sure). All we have to do is make sure the outside pallet is in position, there's and index lever for rotating this pallet and it can only be in one position or it won't rotate on command.
Also, you can program an M36 which works in conjunction with the switch on the pallet changer. M50 will cause the pallet to change regardless, put an M36 in the line before the M50 and the pallet will rotate only after the operator has depressed the switch, indicating that the parts are loaded and the pallet is ready for machining.