https://arstechnica.com/gadgets/2018/06/latest-windows-preview-suggests-youll-be-able-to-turn-s-mode-on-and-off/

Windows 10's S Mode, the locked-down mode that permits only applications from the Microsoft Store, started out as a separate edition of Windows 10 that was locked down as soon as it was installed and had a one-time irreversible upgrade to unrestricted Windows 10 Pro.

In Windows 10 version 1803, that changed to an install-time option for both Windows 10 Home and Pro. Again, this offered an irrevocable upgrade to the corresponding unrestricted version after installation.