Jeff ~ I can tell you my experience the one time I flew CO (sorry this may be a bit long).

I signed up for the OnePass program and never used it ... so about a year later CO sent me two free segment upgrades.

I decided to try and use the upgrades so I was flying MSY-IAH-SNA-IAH-MSY. These were space available upgrades. I was unable to use them on my trip out.

On the trip back I was on a morning flight (probably the 8:45 am flight you are planning on being on also). I got to the airport really early and they were able to put me in first class on the early flight (probably the 6:45 am flight you are thinking about standing by on) with no waiting, but the 8:45 am flight I would have had to waitlist for.

In other words, a peon with no status (me) was able to space available upgrade on the early flight with no problem, so I'd guess that would be your best bet also.

I'd also suggest you check how F looks on both flights using http://www.itn.net, just select the flights you are on (first class) and look how many f seats are available on each flight.