Furthermore, the NW computers appear to have knowledge of your CO status, meaning that you will be able to select priority seating and that your upgrade will be automatically processed (much more effectively than CO's EUA, I might add), even if you log in with a second FF number. I personally didn't find it to be that big of a hassle to sign up for a WorldPerks account that I'll never use. After all, I would have had to create a website login anyway, I'm sure. :-)

Also, according to an NWA gate agent I once asked, NW plat vs. CO plat does -not- make a difference for upgrades; he informed me that they treated CO and NW the same and was surprised when I mentioned that CO gives OnePass members a priority.

Overall, I've had very good success getting upgrades on NW tickets.