Short answer Yes. Modifying RDS Custom is similar to standard Amazon RDS. You can modify the DB instance class and change its DBInstanceClass. This change can be applied immediately or next scheduled maintenance window. But be aware that right now very few DBInstance classes are supported for RDS Custom - https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/custom-reqs-limits.html#custom-reqs-limits.instances.
Check this documentation for details - https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/custom-managing.html#custom-managing.modify-sqlserver
It depends. The general answer is that you can resize the DBInstanceClass. But there are critical caveats depending on the technology.
Modifying an Amazon RDS DB instance indicates this: "You can change the settings of a DB instance to accomplish tasks such as adding additional storage or changing the DB instance class."
But you should also note in Requirements and limitations for Amazon RDS Custom where it indicates, "RDS Custom for Oracle limitations: You can't change the DB instance class, for example from db.m5.xlarge to db.m5.2xlarge."
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