Once you have been migrated to the new format, there is no way to revert to the old one. At some point all Comcast HSI customers will be in the new format.
You have options:
1) Enable the account in a desktop email client like Outlook, Thunderbird, Windows Mail (Windows 10) or some other. That way you can access your account and don't have to look at the Comcast UI. There are a lot of other advantages using a client over only webmail anyway.
2) Set the account to auto-forward to some other, non-Comcast, account you have like Outlook.com, Gmail, Yahoo, Mail.com or some other.
3) Some webmail providers now give you the ability to set up external accounts and use their interface. Notable examples would be Gmail, Outlook.com, and Mail.com. Comcast webmail now has this feature as well.