You can call DescribeValidDBInstanceModifications to learn what modifications you can make to your DB instance. You can use this information when you call ModifyDBInstance.
This command doesn't apply to RDS Custom.
get/#Action=DescribeValidDBInstanceModifications
Query parameters
DBInstanceIdentifierstring required
The customer identifier or the ARN of your DB instance.
Action'DescribeValidDBInstanceModifications' required
Version'2014-10-31' required
Response
Success