(Source: https://stackoverflow.com/questions/54708210/how-to-change-aws-rds-instance-type-without-downtime)
Instead of changing the instance type of the current instance, Add a new instance first. Then, you can delete the old instance in the cluster. There is no way to add the writer instance, So.
In order to change the type of a writer instance.
- Add an Aurora reader with the desired new instance type (for example db.t2.medium) to the cluster. Be sure to assign the reader the best priority so that it will be promoted to writer during failover (see Aurora Documentation for more in-depth information).
- Wait until the new instance is up and running.
- Then failover the cluster so that the new reader gets promoted to a writer.
- Finally delete the previous Aurora instance of the old instance type.
This way you will have nearly no downtime.
If you want to change the type of a reader instance:
- Add an Aurora reader with the desired new instance type.
- Wait until it’s up and running.
- Delete the previous reader instance.