Using Amazon EC2 Capacity Reservation Fleet, you can easily migrate your reserved Amazon EC2 capacity to new generation instance types. Capacity Reservations allow you to reserve capacity for your immediate use in a specific instance type and Availability Zone and can be cancelled by you at any time. With Capacity Reservation Fleet, you can reserve capacity across a prioritized list of instance types. When your reservations for lower priority instance types are unused, it will automatically convert them to capacity reservations for higher priority instance types.
Source:: Amazon AWS