RDS instance class migration and it's impact

Question

We have a MySQL multi-AZ instance on t2 class, which needs to be migrated to t3 family.

AWS documentation says this requires downtime, which is understandable. But it also mentions that only one instance gets modified at a time.

The other instance which is not being modified, can it still provide R/W access to the DB? Because this is how the steps look like:
1. switch over to the standby instance (make it the primary)
2. start converting the old primary to new instance class
3. When the old primary is done, replicate all changes to old primary (this will take some time), promote it to be the new-new primary
4. Modify the secondary, while the new-new primary still handles transactions

Is that a valid understanding or would it be a complete downtime unless both are converted first?

Accepted Answer

Hello.

Even in a multi-AZ configuration, changing the instance type will result in downtime.  
Changes are made from the standby instance.  
Once the changes to the standby instance are complete, perform a failover and make the changes to the other instance as well.  
https://repost.aws/knowledge-center/rds-mysql-downtime-impact  
> Because an instance class change requires a newly defined set of hardware, this change isn't an online operation, and therefore requires downtime. A Multi-AZ deployment of an Amazon RDS MySQL DB instance can significantly reduce any impact. This is because the update doesn't happen simultaneously to the primary and standby. The standby instance is first modified, causing a failover. After the failover, the new standby is modified. The downtime required includes the duration of a failover completion, typically 60-120 seconds,  and completion of the DB engine's crash recovery. For more information, see Multi-AZ deployments.

RDS instance class migration and it's impact

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