Automatically forward email from AT&T / Yahoo
To set up email forwarding from AT&T or Yahoo to your Comcast account, follow the instructions at the following link: https://help.yahoo.com/kb/learn-automatically-forward-email-sln22028.html
Please note that in order for email forwarding to work, your old account will need to remain open. If the account is closed after you have fully transferred to Comcast, email forwarding will no longer be functional. Your best bet is to notify all of your contacts of your new email address.