We're rolling out some changes which should improve your story viewing experience, so if you notice a little toggle switcher on your stories, it means you have early access to the new and improved version.
At first you may not notice too much difference other than a new look and feel. But behind the scenes we've rebuilt stories from the ground up to make things faster and pave the way for some exciting new features.
We're starting with the story viewing experience, and in the next few months rolling it out to the story editor too. So stay tuned!