Change EC2 instance type with SQL HA

Changing the instance type for your SQL servers configured with Windows Server Failover Clustering (WSFC) and High Availability (HA) requires careful planning and execution to ensure the HA configuration remains intact. Here's a step-by-step process to downsize your instances while preserving your HA setup:

1. Backup: Before making any changes, create a full backup of your SQL databases and take snapshots of your EC2 instances.

2. Drain Roles: Move all clustered roles to one node of the cluster.

3. Stop the Cluster: Pause the cluster on the node that is not hosting any roles.

4. Stop Instances: Power off both EC2 instances.

5. Change Instance Type: In the AWS console, change the instance type of both r5d.2xlarge instances to r5d.large.

6. Start Instances: Power on both instances and verify they start correctly.

7. Resume Cluster: Resume the cluster service on both nodes.

8. Test Failover: Perform a test failover to ensure the cluster is functioning correctly.

For the file share witness server, changing from m5.large to t3.medium or c5a.large should not break your SQL HA configuration, as the witness server's role is primarily to maintain quorum. However, follow these steps:

1. Backup the witness server configuration.
2. Stop the witness server instance.
3. Change the instance type to t3.medium or c5a.large.
4. Start the instance and verify it's functioning correctly.
5. Check the WSFC to ensure it recognizes the witness server.

It's important to note that changing instance types, especially to a smaller size, may impact performance. Monitor your SQL servers closely after the change to ensure they can handle the workload with the new instance type. Also, be aware that changing from r5d to a different family (like t3 or c5a for the witness) may require additional steps, such as installing new drivers or making configuration changes.

Always test these changes in a non-production environment first if possible. If you encounter any issues, you can revert to the original instance types using the same process.
Sources
I want to change the instance type "windows with sql server" to "windows standard" for my EC2 | AWS re:Post
Downsize EC2 machines strategy? | AWS re:Post
Change the Instance Type | AWS re:Post