AWS RDS Instance Class Downsizing

Hello.

If the reserved instance period is about to expire, I think it would be better to use an "r6i" instance class instead of "r5".
The "r6i" series instance class has higher performance than the "r5" series instance class.
Other recommendations are as answered by AWS re:Post Agent.
https://aws.amazon.com/rds/oracle/pricing/?nc1=h_ls
https://aws.amazon.com/rds/instance-types/?nc1=h_ls#Memory_Optimized

When downsizing your RDS Oracle instance from db.r5.4xlarge to db.r5.2xlarge, there are a few important considerations to keep in mind:

1. Downtime: Modifying the instance class of an RDS DB instance requires downtime. This is necessary for the change to take effect, so you should plan for a brief interruption in service.

2. Performance: Downsizing from a 4xlarge to a 2xlarge instance will reduce the available compute and memory resources. Ensure that your workload can still perform adequately with the reduced capacity.

3. Endpoint and DB Identifier: The good news is that changing the instance class does not affect your DB instance's endpoint or DB identifier. These will remain the same after the modification.

4. Reserved Instance Renewal: When your current reserved instance expires, you'll need to purchase a new reserved instance for the db.r5.2xlarge class. There's no automatic renewal process, so you'll need to do this manually.

5. Modification Process: To change the instance class, you'll need to modify the DB instance through the AWS Management Console, CLI, or API. This involves selecting the DB instance, choosing "Modify," and then selecting the new instance class.

6. Storage Considerations: While changing the instance class, ensure that your storage configuration is still appropriate for the new instance size.

Remember to test this change in a non-production environment first if possible, to ensure your application performs well with the new instance class. Also, it's a good practice to create a snapshot of your DB instance before making any significant changes.
Sources
Modify the licencing model and database edition of an RDS DB instance | AWS re:Post
Can I schedule an RDS Reserved Instance renewal in advance? | AWS re:Post
Modifying your RDS Custom for Oracle DB instance - Amazon Relational Database Service