your flight would have to be operated by NW crew on NW AC and sold and marketed by NW. you would also have to purchase it from NW and it would have to be ticketed with a NW flight number.

even though it may be DL flight ###, the number will be NW operated or DL operated, specific. US still does this. They have to use specific flight numbers for HP operated flights, as they have yet to combine in to one complete airline. typically HP number are three long and US are four long, on routes where both airlines operate flights.

with all that said, however, id imagine as the official merger draws closer (ie, one operating certificate), CO will lose their upgrade perk on NW and vv. that will probably happen before the merger is complete AND before CO has implemented a similar program (if any) with UA.