No, just a technical requirement since SDC requires both flights (the original and the one being changed to) are within 24 hours. Thus, you have to be able to make two changes to extend 24 hours--one to a flight less than 24 hours away but in the future, then, an hour or more later, you can change to the flight which would have been 24 hours out.

Note that the flights don't necessarily have to be on the same route; if UA serves the destination more than once a day but via different airports, you may, if both routings still are "legal" by UA routing rules, switch that way if needed to make the 2 changes.