What action is required when a military flight changes from VFR to IFR?

When a military flight transitions from Visual Flight Rules (VFR) to Instrument Flight Rules (IFR), the requirement is to update the existing IFR flight plan. This is essential because an IFR flight plan carries specific routing and clearance requirements that must be adhered to for safe navigation in instrument meteorological conditions.

By updating the flight plan, the pilots ensure that air traffic control (ATC) is aware of the change in operations and can adequately manage separation from other aircraft based on the new IFR status. This action allows for the appropriate coordination and provision of air traffic services during the flight, ensuring safety and compliance with airspace regulations.

The correct choice illustrates the principle that transitioning to IFR affects the flight's operational status and necessitates a formal update to its flight plan, aligning it with the appropriate procedural standards for instrument flight.