How should military flights indicate a change in destination when communicating with FSS?

Updating the flight plan is the correct approach for military flights to indicate a change in destination when communicating with Flight Service Stations (FSS). This process ensures that all relevant authorities have the most current information regarding the flight. An updated flight plan helps maintain operational safety, enables better traffic management, and ensures that rescue or search and recovery operations have accurate information in case of an incident.

When a military flight alters its destination, it is crucial to provide that updated information through the proper channels. This traceability is vital, as it allows other agencies and air traffic control to be aware of the flight's intentions and current route, thus reducing the risk of confusion or conflicts with other airspace users.

The other options, while they may contain some elements of communication, do not fulfill the requirement as completely or effectively as updating the flight plan. A brief summary may lack the necessary detail to relay all changes and intentions. Using different communication channels might complicate the process without ensuring that all necessary parties are notified. Calling in upon arrival does not inform FSS of the change in plans ahead of time, which could lead to safety concerns.