Yes, you can change the user ID-----that portion of the email address to the left of the @ symbol. On the Xfinity home page, click on "My Account" at the top. Sign in again. On the top of the landing page, click on "Users". The account name should now show. To the right, click on "Edit", and follow the prompts.
Caution-----once you change the user ID, you CANNOT get the old one back. So make sure the new one is really what you want.