It depends upon the flight times. As long as the flight you want departs less than 24 hours prior to the one that you have, it’s eligible for SDC, as soon as you reach the magical 24 hour window. So, if you’re booked on 2 PM departure Friday, at 2 PM Thursday, you can theoretically rebook to any flight the rest of the day Thursday.

If you have Premier status with UA, and there are seats available in the same fare bucket that you have now, this is free. If not, you’ll need to pay a fare difference based upon the target flight’s inventory.