I don't know about NW but I just got off the phone with Delta..The agent was wonderful!
I asked if and what charges would I have if I changed my flight days and times..She said of course the 50.00 on Delta Express for all 3 tickets=150 plus the difference of the fares about 134.00 each, but if I took a flight out at 11am that same day instead of 5:00pm no charge! Because my daughter has school the next day, I thought arriving home earlier would work better for us, instead of getting in a 8:30 and driving another 1 1/2 hours till we got home..
Of course she thought she'd miss another day of school..sorry honey!!
Right now I think all the airlines will work with you..Best of Luck