I have tried this recently and has worked both times. If you definately would like an upgrade and are somewhat flexible with your plans, CO will let you take another flight (assuming First is already full on your scheduled flight) with some conditions.

For example, On a recent flight from Altanta to Neward I was schedules on a 6:45AM flight. Since I am only Bronze and using ELTS I called the night before to see about availablilty and they said that the 6:46 was full in first. I asked when the next flight was and they said that the 8:45 had 5 open seats. According to CO their policy is if you "miss" a flight, and check in within 2 hours after the departure, you can take the next flight with no penelty. This worked well for me and even got to sleep an hour later.

Many of you may know this but if you fly on restricted tickets and are not gold or platinum it is sometimes worth it.. Just my 2 cents.