When attempting to change the instance type from t3.small.search to t3.medium.search on a development domain in Amazon OpenSearch Service, the following message was displayed.

Autotune is not supported in t2/t3 instance types. Disable autotune or change your instance type.

I could not find the Autotune setting in the UI, but the following page described how to do it using the CLI.

Auto-Tune for Amazon OpenSearch Service - Amazon OpenSearch Service
Learn how to use Auto-Tune for Amazon OpenSearch Service to optimize your cluster performance.
docs.aws.amazon.com

So I ran the following command.

aws opensearch update-domain-config \
  --domain-name my-domain \
  --auto-tune-options DesiredState=DISABLED

After that, I was able to successfully change the instance type.

I am not fully aware of the disadvantages of disabling or not using the Autotune feature, but I hope this is helpful for anyone facing the same issue.