I've done this a few times. You are subject to a $25 fee and the rest is issued in an ECV, Electronic Credit Voucher. I've found now that I can go in and view my reservations on nwa.com. Click to change the reservation, go through the steps of choosing the same flights, same days, and it will show you the new price, show the difference in fare, take out the $25 and then click confirm and it will spit out a confirmation of your credit.