For all reschedules of the event you should only do so when a customer has requested to change the date or agreed to change the date. If they have requested or agreed to a new date you can reschedule the event for them either from the message thread or the event details page.
Message Thread
1) Select reschedule from the actions on the right hand side
2) Choose their requested date and time from the date widget. Then select the check box confirming that the date and time change has been approved by the customer and click submit.
Event Detail Page
1) Select the Event Actions dropdown and choose reschedule from the available actions.
2) Choose their requested date and time from the date widget. Then select the check box confirming that the date and time change has been approved by the customer and click submit.