Outlook allows you to have multiple profiles which means you can simply add the old Comcast account to Outlook and enable syncing through POP/IMAP. There is also a page provided by the Comcast with instructions about this procedure. Once you have all the email in the Outlook, add another account (the new one) and move the files from one profile to another.
Make sure to stay on the track regarding settings on that page. If you write something unnecessary, the process won't work normally.
Comcast page: http://customer.comcast.com/help-and-support/internet/configure-outlook-2003-xfinity-email
To make it even easier, use the TransferMyEmail service provided by Comcast.