Also noteworthy is that when you enter your CO FF# into the NW website, it won't recognize your elite status until after you buy the ticket. This means you'll have to choose regular seats right at first, but you can go back a couple of minutes later and it will have picked up your skyteam elite status and let you take any premium seat incuding exit rows. Seats marked Coach choice can't be reserved by anyone until 24h before the flight at OLCI, Skyteam elites get them free, non elites have to pay for them.

You'll most likely get upgraded for the flight, NW is generally far easier to get upgraded on than CO. My status is on CO too, but my upgrade % on NW is still 100% to date. Keep checking your reservation starting about a week out to see if you've been EUA'd, this way you can avoid the bulkhead in first (if desired) and choose aisle or window.