Rename the Amazon RDS master username

You can't change username. You can check the following links that describe how to change master password and if Amazon adds the ability to change username you will find there:

Try to find at AWS CLI for RDS:

 modify-db-instance --db-instance-identifier <value> --master-user-password (string)

--master-user-password (string)

The new password for the DB instance master user. Can be any printable ASCII character except "/", """, or "@".

Changing this parameter does not result in an outage and the change is asynchronously applied as soon as possible. Between the time of the request and the completion of the request, the MasterUserPassword element exists in the PendingModifiedValues element of the operation response. Default: Uses existing setting

Constraints: Must be 8 to 41 alphanumeric characters (MySQL, MariaDB, and Amazon Aurora), 8 to 30 alphanumeric characters (Oracle), or 8 to 128 alphanumeric characters (SQL Server).

The Amazon RDS Command Line Interface (CLI) has been deprecated. Instead, use the AWS CLI for RDS.

Via the AWS Management Console, choose the instance you need to reset the password for, click ‘Modify’ then choose a new master password.

If you don’t want to use the AWS Console, you can use the rds-modify-db-instance command (as per Amazon’s documentation for RDS) to reset it directly, given the AWS command line tools: rds-modify-db-instance instance-name --master-user-password examplepassword

No. As of April 2019 one cannot reset the 'master username'.

You cannot do it directly. However you can use the database migration service from AWS:

https://aws.amazon.com/dms/

Essentially you define the current database instance as your source and the new database with the correct username as your target of the migration.

This way you migrate the data from one to another database instance. As such you can change all properties including the username.

This approach has some drawbacks:

• You need to configure the migration. Which takes a bit of time.
• The data is migrated. This may lead unexpected behavior since not everything is eventually migrated (e.g. views etc.)
• It depends how you setup everything you may experience a downtime.