instanceMatchCriteria

The matching criteria (instance eligibility) that you want to use in the modified Capacity Reservation. If you change the instance eligibility of an existing Capacity Reservation from targeted to open, any running instances that match the attributes of the Capacity Reservation, have the CapacityReservationPreference set to open, and are not yet running in the Capacity Reservation, will automatically use the modified Capacity Reservation.

To modify the instance eligibility, the Capacity Reservation must be completely idle (zero usage).