generally, upgrade certificates are limited by carriers if bookings look strong. in the case of CO, they are next to impossible to use (and that situation will probably continue when it goes "certificate-less"). the real problem is that unless you are in the top category (chairman's, 1K, platinum, etc.) the airlines are just not making the inventory available. this means you are having to do the thing at the gate, and that is often based on what tme you get there. sometimes you just can't be early for a departure. I am chairman's preferred on US and do well on upgrading. my wife holds only preferred status with them. when she recently tried to upgrade 24 hours in advance by phone, she was told no seats available in F. I looked in a computer program I have which shows seat inventory, and they had 5 of 12 still unsold!