At 36 hours to schedule departure all seats open up on NWA.COM checkin
A lot of pax improve their seat assignment at appr. 36 hours prior to scheduled departure time.
I have also heard elite line agents advise that that is when to look for a better seat for a non-elite in the elite section (unavailable to non-elites until e-checkin opens up at 36 hours to flight)
Good Luck!
Rc