http://www.techradar.com/news/microsoft-gives-you-three-extra-months-to-upgrade-from-windows-10-s-for-free

Microsoft has announced that it's extending by three months its free upgrade period for people switching from Windows 10 S to Windows 10 Pro.

Originally, Microsoft was only offering free upgrades until December 31, 2017, but now you’ll be able to swap Windows 10 S for Windows 10 Pro for free until March 31, 2018.