Originally Posted by
schley
A some day confirmed change is only available on the day of travel for your original ticket. ....
That is true for UA's SDC. CO allows changes to within 24 hours (before / after) your scheduled flighttime. The rules are still separate. What the OP desires to do can be done on a CO operated flight (assuming seat availability) but not on a UA operated flight.
So the question for the OP, who is the operator of your flight (operator, not who sold / marketed the flight)?