Hello,
I think your best bet will be to download the emails from comcast into a program on your computer. Move all of your email into your comcast inbox. Then configure a program such as
Mozilla Thunderbird to access the comcast email as a pop3 account. The incoming server name is mail.comcast.net and the port is 110.
If you get the Yahoo account before you lose access to the Comcast account you can try forwarding all of the email in batches to your new Yahoo based ATT account and sorting it into the folders structure you'd like as they arrive.
A third option, and probably your best long term bet, would be to open a Gmail account and forward all of your email there. You should be able to forward most email accounts there and Gmail can use POP3/IMAP to access the rest. It will help you to avoid problems like moving from Comcast to ATT the next time you need to switch internet providers.
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