Yeah, once you have your DL number in the reservation, it often comes back up despite having changed it to your NW number. You have to have the person physically delete the DL number out of the system as oppose to just adding your NW number. Always double check your boarding pass since that's the most important part. They have been able to change that for me, though with some work usually at the gate and print out new BPs. They should be able to do this as long as it's before the flight, even mid trip.

So you don't get any bonus miles on DL if you get NW miles, BTW.