How do I verify my account with the Yahoo sign-in button?
This AT&T/Yahoo issue only affects address book migrations. Try starting an email-only migration by unselecting the "Contacts" button in step 3 on the start page.
You can transfer your contacts between your old account and your new account using the new Contacts Import Portal. Follow these steps to transfer from your AT&T/Yahoo account: http://comcast-contacts-portal.helpscoutdocs.com/article/215-walkthrough
Once you have transferred your address book, you can then notify your contacts of your email address change by following the steps here (beginning with STEP 1, located below the first two photos).