I just called CO. The lady on the phone said this:
For the same day change policy, you can only change your flight to one that departs on the same day; Not within 24 hours.
I think she's half wrong.
From CO's website in the FAQ for SDC policy:
Can I change my flight to the day before or the day after?
Yes, but only when you make the change within 24 hours before your original scheduled departure, and your new itinerary departs within 12 hours of the request.
Either way, in order to fly tonight. I have to wait until after 5:50pm to make the request.