Changing Opensearch instance type - blue / green deployment

Based on the dry run output you've shared, you are partially correct, but there are some important nuances to consider.

The output indicates that the changes you're attempting to make will require a blue/green deployment. This means that OpenSearch Service will create a new environment with your requested changes, and then migrate your data to this new environment.

During a blue/green deployment, your existing cluster (the "blue" environment) will indeed remain online and usable. This ensures that your service continues to operate without interruption. However, it's important to note that while the existing cluster remains functional, there may be some performance impact due to the additional resources being used for the deployment process.

The new environment (the "green" environment) will be created alongside your existing one. Once it's ready and data migration is complete, OpenSearch Service will switch traffic to the new environment. This switchover is designed to be seamless, but there might be a brief moment of transition.

Regarding configuration changes, you are correct that during the blue/green deployment process, additional configuration changes will be restricted. This is to ensure the integrity and consistency of the deployment process.

It's worth noting that blue/green deployments are resource-intensive and can take some time to complete. For this reason, it's generally recommended to schedule such changes during off-peak hours to minimize any potential impact on your operations.

Also, recent updates to OpenSearch Service have made it possible to perform certain types of changes without requiring a blue/green deployment, such as modifying dedicated cluster manager nodes or updating cluster volume settings (for specific conditions). However, based on your dry run output, it appears that the changes you're attempting still require the blue/green approach.
Sources
DryRunResults - Amazon OpenSearch Service
Auto-Tune for Amazon OpenSearch Service - Amazon OpenSearch Service
Amazon OpenSearch H2 2023 in review | AWS Big Data Blog
Amazon OpenSearch Service now lets you update cluster volume without blue/green