NWA is pretty much the only airline that operates anywhere near me, and they do this all the time. Particularly since their bankruptcy, they are constantly changing flight schedules--allegedly to save money. A couple years ago I flew out to New York, and the schedule changed seven times between buying the tickets and taking the trip.
In most cases the change isn't terribly important (a few minutes here or there). If it appears to give you too tight of a connection in a hub, though, call the airline and see if they can arrange another itinerary. On my trip out east the final schedule change would have made a 30-minute connection in Minneapolis, but they were able to fix it fairly easily.
The good news is that Northwest is usually quite generous in the flying time they allow, so you're less likely to arrive late than on other airlines.