Final answer:
The appropriate air traffic control (ATC) unit should be notified when changes are made to a flight plan or flight itinerary to maintain safe and efficient airspace operations.
Step-by-step explanation:
When there are changes to the information in a flight plan or flight itinerary, it is crucial that the appropriate air traffic control (ATC) unit be notified. This could be the departure, destination, or en-route ATC, depending on the timing and nature of the change. Pilots may inform ATC of changes through radio communication or by contacting the relevant flight service station. It is important to ensure that ATC has the most current information to maintain the safety and efficiency of the airspace system.