I would say it is very, very unlikely. Notwithstanding the availability of 'Z' class seats, there is a no-upgrade within three days of the flight rule that CO is pretty serious about, and day of flight is, well, you see what I'm getting at. Add in the fact that you're on a reward ticket to begin with and I would say your chances are basically nil.

That having been said, "it never hurts to ask." But the next obstacle is, as far as I know, CO will not send you a certificate simply 'cause you'd like to have one handy-- they do most upgrades electronically at this point anyway.

If you have a certificate handy, bring it, but it is very, very unlikely you will have the opportunity to use it. Enjoy your trip! -jp