Reserved Instance Modifying Request

Hello.

Unfortunately, you cannot cancel a reserved instance or change the instance type applied to it later.
Therefore, I recommend that you operate without changing the instance class until the current reserved instance term ends, and then change it after the reserved instance term ends.
https://docs.aws.amazon.com/opensearch-service/latest/developerguide/ri.html

RIs are not flexible; they only apply to the exact instance type that you reserve. For example, a reservation for eight c5.2xlarge.search instances does not apply to sixteen c5.xlarge.search instances or four c5.4xlarge.search instances. However, linked accounts that are part of an organization in AWS Organizations can benefit from any unused discount application from the owning account of the RI as long as the instance types, Region, family, and size match. For more information, see Amazon OpenSearch Service pricing and FAQ.