I had kind of the opposite thing happen a couple of weeks ago ORD-TPA. CO rebooked me onto the UA 530p nonstop-it was sold out but CO asked me "do you have certificates to upgrade?" Mystified, I replied "sure do" then walked over to UA. They said that at the beginning of the day the 1c seats which are unsold become available for upgrading (they don't sell much 1c into/out of Florida)and I lucked into one of the seats that was an upgrade seat. This was at the PE/1k desk at ORD...