Microsoft announce Windows 10 S – a flavour of Windows 10 Pro that is, in Microsoft’s words, “optimised for security and performance”. It does this by working only with apps from the Windows Store and having Edge as its default browser. You can pay to switch to Windows 10 Pro through the Windows Store at any time, but you can’t then go back to Windows 10 S.

This FAQ gives you a good overview: http://bit.ly/2pB32Pt.