When I decided to leave Comcast (ended up going back) a few years ago, I realized what a mistake I had made by using my Comcast email as a primary account. I will never make that mistake again. As someone else suggested, get something like a Gmail account.
If I remember correctly, you have access to your Comcast email for a month or so after canceling your account. Before you cancel, set up another email account and export all of your contacts to the new email account. Start using your new account right away, and set it up so your Comcast emails are forwarded to the new account. The forwarding will only last until they shut off your access to the Comcast email account.
I'm sure you've already thought about it, but you will need to update every account (bank, ebay, paypal, credit cards, bills, websites, etc) associated with the Comcast email. It was a pain.