Its not uncommon for meetings to be rescheduled last minute. There's a current limitation in Webex that prevents meetings to be rescheduled if a single participant has joined the scheduled meeting; the host will receive an error message stating the meeting is 'Already in progress' and cannot make changes to the meeting until all participants have left.
Can the ability to reschedule meetings while the meeting is in progress please be enabled. Additionally, an in-meeting prompt stating the "The meeting has been rescheduled and will end in XX minutes, please contact the Host for more information" would be a good compliment feature.
Currently, our architecture does not allow us to do this. We are evaluating changes to the underlying architecute that will hopefully help in achieving this.