Microsoft recently released an all-new Surface Laptop running Windows 10 S out of the box -- an OS that only runs apps downloaded from the Windows Store. This makes the Surface Laptop an expensive competitor to Google's Chromebook, although more affordable Windows 10 S laptops from HP, Acer, and others are expected by the end of the year.
As an extra incentive for early buyers, Microsoft is offering free Windows 10 Pro upgrades for Surface Laptop users until the end of the year, after which the upgrades will cost $50. Though Microsoft had said this upgrade would be a one-way street (citing security concerns), it seems that they have changed their stance. It's now possible for Surface Laptop owners to revert back to Windows 10 S from a Pro upgrade.
Unfortunately, that's easier said than done. The process of reverting is not as simple as the process of upgrading, which usually takes less than 60 seconds and requires one reboot. Reverting to Windows 10 S will require a complete system wipe using one of Microsoft's official recovery images, which can be downloaded from the official Surface website (they are about 9GB in size). While the situation isn't ideal, it's the only method of reverting available at the moment.