I am not certain what you mean by downgrade. Did you mean substitute one kind of aircraft for another (757-200 for A320 for example) or simply that you feel the aircraft you had was not up there in terms of standards.

The older 757-200 have limited carry-on storage space and that will not change until they are refurbished. Right now I would think that NW is more worried about getting out of Chapter 11. In time there will be upgrading the interiors. BTW, it flies just fine...