Requirements for Modification
Amazon EC2 processes your modification request if there is sufficient capacity for your target configuration (if applicable), and if the following conditions are met.
Your modified Reserved Instances must be:
Not pending another modification request
Not listed in the Reserved Instance Marketplace
Terminating in the same hour (but not minutes or seconds)
Your modification request must be:
A unique combination of scope, instance type, instance size, offering class, and network platform attributes
A match between the instance size footprint of the active reservation and the target configuration
Only Standard Reserved Instances can be modified.
If your Reserved Instances are not in the active state or cannot be modified, the Modify Reserved Instances button in the AWS Management Console is not enabled. If you select multiple Reserved Instances for modification and one or more are for a platform that does not allow instance type modification, the Modify Reserved Instances page does not show the option of changing the instance type of any of the selected Reserved Instances. For more information, see Modifying the Instance Size of Your Reservations.
You may modify your reservations as frequently as you like; however, you cannot submit a modification request for reservations that are pending a previous modification request. Also, you cannot change or cancel a pending modification request after you submit it. After the modification has completed successfully, you can submit another modification request to roll back any changes you made. For more information, see Determining the Status of Your Modification.
To modify Reserved Instances that are listed in the Reserved Instance Marketplace, cancel the listing, request modification, and then list them again. In addition, you cannot modify an offering before or at the same time that you purchase it. For more information, see Reserved Instance Marketplace.