Downgrade RDS Instance size

It is the same process as choosing a larger instance type.

It is very easy. You can just call modify-db-instance to scale up and then after your test just call modify-db-instance to scale down.

You can also create a blue green deployment that will clone your environment. Then you can scale the green instance, run your tests, and then delete green and the blue green deployment.

https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/blue-green-deployments-overview.html