I respectfully disagree. I've done it a few times when I had the "Trial CO Silver" and wanted to earn NW miles. Just have them do it at the gate or get both the number in there. Call the Res line and have them enter your DL number in there and ask them to keep your CO number in their too so you can check-in online. They may say that they can't do this, but they've done it for me 2x before, so it's possible. Check-in w/ you CO number and get the upgrade and change it at the gate as close to boarding as possible (at this point, your seat has been assigned, and unless it's a really oversold flight, you should retain your seat...)