I've been in this position before and NW was able to reaccomodate me. You shouldn't be restricted to award inventory since you booked a valid res and schedule changes are the problem. That said, how far out is your flight? If it's a while from now, you may see some more schedule changes -- it could be worth waiting until you're within 4-6 weeks of the flight to have them redo everything, or you'll just be in the position of going through this again.

For what it's worth, when it happened to me on a biz class award ticket out of IAH to CDG (and connecting to MRU from there), they went so far as to put me on PrivatAir IAH-AMS and then a city hopper AMS-CDG to make my CDG-MRU connection. So as far as I'm concerned, they can do pretty much whatever they're inclined to do. That was done by a ticketing agent at IAH day-of, when I think they have more authority/latitude.

peace,
~Ben~