Call them back and switch it with your NW FF# there is no need to
collect miles on multiple FF accounts.. would be much easier to use only NW
in your case.

When you call, make sure you tell them to

1. remove your delta FF number from your reservation
2. then add your NW FF number to your reservation.

and make sure when you check in.. they have your NW FF number
on the boarding pass.

- dont count on any upgrades on DL.