Updates an existing calendar event. publication_id is required but must match the event's current publication; calendar_event_category_id is also required and must match the event's current category, and channel, when given, must match the event's current channel. None of them can be changed after the event is created. While the event has a message waiting for approval or approved but unsent, start_date_time is locked because the approval covers the send time; withdraw the message (withdraw_calendar_event_message) to move the event, then send it for approval again.