Currently in ServiceDesk 7.1 Service Pack 2 & 7.5 when a Change Ticket gets approved a calendar entry is created. If after this entry is created it is decided that the schedule for the change needs to be modified and a user updates the calendar entry to reflect the new date the Change Management ticket isn't updated to reflect this change.
No functionality was provided with ServiceDesk to monitor the calendar entries created by the Change Management process. The only information saved in the database for the calendar entries to correlate the Change Management tickets & the specific entries in the text [CM-######] that appears in the entry name.
To work around this limitation you can create a CalendarEventListener project (found in the templates tab) to monitor for any calendar entry modifications and check to see if the entry was for a CM ticket.
I've created and attached a sample 7.5 project.