You're allowed to re-checkin at the kiosk in IAD and change your seats, so I think you'd be able to do that at the main kiosks as well but I've never tried.
I did notice that the exit row seats on my A319 flight recently seemed to suddenly become available 30 days prior to the flight, though I don't know if that was something about my specific flight or if that's a normal policy. They were completely booked before that but opened up most of them 29-30 days before (could have also been some cancellations).