Another data point. (AKA "I wish I'd read this thread earlier.") As a CO Platinum and One Pass Plus cardholder, about a week before the return leg of a RT flight, I received an email notice that the leg had been upgraded. I completed the first leg then, at the airport for the return leg, chose to change the return leg to an earlier flight that same day. I asked about upgrades at the gate and was told that no upgrades were possible on an award ticket and that whomever had given me the upgrade previously "did not do their homework" on the rules. Since I had not read this thread I did not mention that I was a ONE+ cardholder but this agent seemed to believe that no upgrade was possible on an award ticket under any circumstances.