Back in September, Twitter rolled out new Facebook-like profiles that featured header photos. Some people switched to the new style immediately, while others opted to stick with what they had. Well, to those who are fighting change, your days are numbered. Twitter today announced on its blog that on Wednesday, December 12 everyone's profiles will move to the new style.
In the blog post, Twitter encourages everyone to be proactive about the change and pick out a header image lest their profile looks bare and plain:
On December 12, we’re rolling this out to all users: you’ll automatically get this new version of the profile on twitter.com. If you don’t upload a header photo by then, you (and everyone else) will only see a default grey image on your page. That’s not fun! To get inspired about what you can do, check out this video to see how to make your profile a little more “you”, less generic. Have fun out there.
If you're still confused about how this will all work, Twitter produced a short animated promotional video that explains the change.