Microsoft is releasing a new Office 365 sign in experience to end users, a change that has not been communicated on the roadmap, Office Blog, or in Message Center. Upon visiting the login page for Office 365, users are shown a popup that will take them to the new sign-in experience.
Update: Microsoft has posted an announcement to the EMS blog, which is the wrong place to announce this kind of change, and has been posted after the change rolled out to customers.
The new sign-in page is functionally the same as the old one but uses a different layout. On some of my visits a background image has been displayed, but on most visits so far the background has been blank. It's unknown whether the background will be a single image, or a Bing-style rotating gallery of images.
The new layout will play havoc with any custom branding that has been specifically designed for the placement of page elements in the old sign-in experience. If you use custom branding for your login pages I would recommend you start working on the necessary adjustments now, and try for a design that works in both layouts.
The popup suggesting to try the new sign-in experience may or may not be related to First Release tenants. The popup appears pre-login and also appears for me in private mode browser sessions, although it is not appearing for me when tested with a ToR browser. Perhaps the rollout is limited to certain geographic regions. There's no administrative control I can see for disabling the popup.
A fresh new look is all well and good. The old sign-in page has some annoying UX quirks when I visit it in a Chrome browser. The new one seems to behave much better, at least for now. After you try it the first time you will continue to be taken to the new sign-in experience on subsequent visits, presumably due to a cookie being set in your browser. You can switch back at any time and that change will stick as well.
As an unannounced change this has naturally caused some concern among customers. Which is not surprising. A similar change back in April caused quite a stir, with one Microsoft representative pledging to do better in future.
The concerns are valid. When users are surprised it usually ends up creating a spike in support calls. Customers also put considerable effort into training their users to treat such changes as potential phishing attacks. Being able to tell users about a change beforehand helps to smooth the transition. At the very least, anyone who invests in internal training materials for their users would appreciate some warning so they can update their documents accordingly.
Communication is key.
Update: see the comments below for reports of what the new sign-in experience breaks.