I'd print it out for reference, but if you are asking about what you need to bring with you to the airport, either the receipt or a copy of the itinerary or the change notice NWA sent will be fine.
NWA has all your pertinent info in its computer. You'll just need a photo ID to check in. I usually just hand them my ID and say that I have an e-TICKET to Orlando. If I remember the flight number, I'll also tell them that. The agent always finds me (and my party). I do keep a copy of the receipt and/or itinerary with me, just in case, but have never needed it.
Since 9/11, however, you will need a copy of the itinerary or receipt (stuff you originally printed from the computer) or the mail NWA sent you for the flight change to get past security if you check in at the gate (because you either have no luggage to check or you checked it curbside). If you check in at the counter, you will have a boarding pass which will get you through security.
I'm sure that's way more than you really wanted to know, so I hope something I said actually answered your question.
Enjoy your trip!