I the US stage IV only applies to new aircraft. NW can keep flying the DC-9 as long as they deem fit. The DC-9 and the MD-88 also share a lot of parts. The biggest differance is the cockpit instruments. So IF they were to merge I'll bet both keep flying until either new 737-NG or Airbus A320 deliveries can begin. At that point the one not taken would be dumped with the MD products. I bet the 767 would stick around until the 787 comes on line. I would bet the 777 and larger Airbus would be sold and the 747 be kept for the Asian markets.