Does RDS Reserved Instance Apply After Scaling Instance Type?

Hello.

Reserved Instances are applied automatically.
However, since it was purchased for db.t3.large, it cannot cover the entire db.t3.xlarge.
As stated in the documentation below, the normalization unit for large is 4 and for xlarge it is 8.
Therefore, the remaining 4 will be charged at the On-Demand Instance rate.
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_WorkingWithReservedDBInstances.html#USER_WorkingWithReservedDBInstances.SizeFlexible

If you have a DB instance, and you need to scale it to larger capacity, your reserved DB instance is automatically applied to your scaled DB instance. That is, your reserved DB instances are automatically applied across all DB instance class sizes. Size-flexible reserved DB instances are available for DB instances with the same AWS Region and database engine. Size-flexible reserved DB instances can only scale in their instance class type. For example, a reserved DB instance for a db.r5.large can apply to a db.r5.xlarge, but not to a db.r6g.large, because db.r5 and db.r6g are different instance class types.

When you purchase a Reserved Instance (RI) for Amazon RDS, the RI discount applies to a specific instance class, region, database engine, and other characteristics, but you do have some flexibility depending on the type of RI you have purchased. For Standard Reserved Instances, the discount only applies to the exact instance type and size you originally reserved. Therefore, if you change your RDS instance size from, for example, db.t3.large to db.t3.xlarge even within the same instance family, the existing Reserved Instance discount will no longer automatically apply to the new, larger instance size. In this scenario, the RI will remain unused until you launch another db. db.t3.large instance. You do not need to manually change any filters or configurations in the Reserved Instance itself, but the discount won’t follow your scaled instance.

However, if you are using Convertible Reserved Instances, you have more flexibility. Convertible RIs allow you to exchange your reservation for another one with a different instance size, family, or even database engine, as long as the exchange is for reservation(s) of equal or greater value. You must perform this conversion manually through the AWS Management Console, and the discount will then apply to the new configuration.

Official AWS documentation confirms this behavior:

• For Standard RIs, “You can’t change an instance size within the same instance family; the reserved pricing applies to the specific instance size”… “If you launch a different instance size, the Reserved Instance pricing doesn’t apply” (see Amazon RDS Reserved Instances).
• For Convertible RIs: “You can exchange a Convertible Reserved Instance for another Convertible Reserved Instance with different instance attributes, including instance family, instance type, engine, or tenancy” ([AWS RDS RI documentation].

In summary, after scaling your RDS instance within the same family but to a larger size, a Standard Reserved Instance will not apply automatically to your new instance size, nor can it be changed to do so. Only Convertible Reserved Instances offer the flexibility to switch and require a manual exchange. For further details, refer directly to AWS’s official documentation links above.