There are more friendly tools for renaming Google Apps users that you can purchase but GAM (Google Apps Manager) is free and can do user renaming plus much more:
http://code.google.com/p/google-apps-manager/
It's command line driven. We use it for various batch operations like turning off webclips for all Google Apps user accounts.
With the ability to rename users there is no need to backup and restore user accounts just to change their email address.