It is pretty easy to fix the backwards roof hatch on the new Lionel GS-4s.
Remove the body from the chassis.
Loosen the metal clip on the underside of the boiler to give slack to the wires feeding the cab light.
Unscrew the cab light from the roof, now you can work on the cab freely.
The roof hatch and its frame are held onto the cab roof by 4 wire prongs. Straighten the prongs and pull the hatch assembly up and off the cab room.
Turn it 180 and bend the prongs back down.
NO big deal.