You have come to the right place, first, you will want to go sign up for a Start Mail account.
Then you should start applying your new email address to every account using your existing email address.
Then you will configure your existing email account on Start Mail and then copy your mail between the two accounts.
You can click or tap here to find a guide from Start Mail, they really make the transition process as easy as possible.
Click or tap here for your imap setting for Comcast when adding your Comcast email to Start Mail