This REST endpoint only allows to set the initial username. Once set the username cannot be changed or deleted. Use the new REST endpoint from the UI instead of the old AccountSecurity.changeUserName(...) RPC. The AccountSecurity.changeUserName(...) RPC is removed since it is no longer used. Change-Id: I48f4d7642b551e17ceef7772563a229aa83fc1ad Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>