Audie:

From my experiece with NWA.com, I have been able to chage seat assignments only for tickets I booked on NWA.com. NWA.com will show me what seats I have on flights booked via other means (i.e. travel agent, MTL vacations, or via NWA on the phone) but I have not been able to change my seat assignements on the web. I've had to call NWA.

Hope this helps