Once you have been migrated, there is no way to revert to the old format. At some point, all customers will be in the new format.
The new format is designed more with mobile devices in mind. But it also functions with people using a desktop or laptop computer. Since nearly 50% of all logins to Comcast are now done with mobile devices, this was pretty much inevitable. It looks very much like an iCloud or Outlook.com account. They've been rolling it out for over a year and a half now.
Well..it doesn't work very well on Mobile devices either. Didn't work on the Desktop, Kindle, or Smartphone the other day when that new format was having issues. Too bad Comcast doesn't just admit it is a bad design and go back to the old format. Instead we get to suffer this mess.