As a general rule, you can only keep your email address with an ISP if you continue to pay for service. You might be able to downgrade your service level, but the least expensive service that I see AT&T offering is $19.95/month, which is a lot to pay just to keep your email address (you can get your own domain with email service for less than that per year from a place like 1and1.com).
What I've typically done when I was changing ISPs is pay for service on both for a month or so, to give time for the email changeover (though finally I got too annoyed with the nuisance factor of changing addresses, and I now have an email address that's independent of my ISP). I'd send out a general email to all my contacts letting them know of the new address, then over the course of the month, I'd check both email addresses, sending reminders (or going to websites to update my address) whenever I got email to the old address.
