If you know your credentials (username/pwd), you can log in, and change phone number, the one associated with the account, and you're all set.
If not, and you have access to the new email now, then you can reset your password that way. Then you can log in, and do the same (change to the new contact info, same deal).
If you don't have any of these (which would seem odd), then you'll have to contact AT&T, and "prove" to them who you are, and they'll reset your credentials. They usually have several other "challenge" info questions, with which to do this.
