There are several significant differences between UA & CO SDC programs.

Online at UA (or by phone) you can only do SDC within 3 hours of the new flight and only for identical routing. If inventory is available in H class, you can be confirmed regardless of fare class paid (free if 1P or above, otherwise fee is charged.) If no inventory is available you can stand by. At the airport via an agent they seem to offer routing flexibility.

On CO you can SDC +/- 24 hours from the time you are making the request, which can only be done via the check-in application, so that starts 24 hours before your flight. At that time you can switch to anything earlier with inventory in your ticketed fare class. As you get closer to your flight time, you can also change to flights later than your flight if inventory is available in yoru tickted fare class. The routing does NOT need to be identical, but the SDC application seems to only show limited choices, perhaps because of the inventory class restriction. In practice, the CO approach is more useful. However the caveat is that often the cheaper fare categories are not opened until 3 or 4 hours before the flight, which makes it work a lot more like the UA program on any flight that's close to full. Also the CO website doesn't easily let you see fare class inventory, but there are other tools out there which can show it.

It remains to be seen how UA & CO will merge SDC. I certainly hope they will adopt the CO 24-hour window and routing flexbility.