In order to change a user's password when the clients are distributed and the system is under production load, it is useful to be able to manage multiple passwords per user, to enable a phased change.
- create new password
- change clients configuration
- revoke old password
Amazon describes this process here: http://docs.amazonwebservices.com/AWSSecurityCredentials/1.0/AboutAWSCredentials.html#CredentialRotation