Using the AWS Management Console, you can now save an environment’s configuration, launch a new environment with a saved configuration, and apply a saved configuration to an existing environment. This allows you to seamlessly launch multiple environments with your preferred settings.

AND

By using the URL swapping functionality, end users of your web application should not experience downtime when you deploy a new application version. You can stage your new application version on a test environment and swap the URLs with the production environment.

Read more at:
http://aws.typepad.com/aws/2011/06/configuration-persistence-and-support-for-url-swapping-with-aws-elastic-beanstalk.html