Can I Encrypt the Disk After Purchasing an RDS DB Instance?
Sorry, but you cannot. Once the DB instance is created, you cannot modify the disk encryption status or change the key. The backup data stored in Object Storage Service (OBS) is not encrypted.
To encrypt the disk for a created DB instance, you can restore the DB instance from the backup data to a new DB instance and then enable disk encryption for the new DB instance. Alternatively, use DRS to migrate data from the created DB instance to a DB instance with the disk encrypted.