This has happened to me as well - just make sure you have both types of upgrades in your possession upon checkin - if they call your "bluff" and you forget to bring the real thing, or do not have the real thing entirely, you could find yourself in a middle seat in coach, or worse, off the flight entirely if it is overbooked!

This is a game that some people do play, but I have heard stories that make me want to be the honest guy at check-in.

SFOPremex