You know when you first use your phone to get an email for BIS you have to create a username and password, can you change your username at all. I noticed you can change the password but can't seem to find a place to change my user name.
You can delete the email address and add it back on with a different user name. Just make sure that you delete the service books once you've deleted it from BIS on your blackberry.
To delete the service books, go to options -> Advanced -> Service Books. Then delete the old ones.