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.
The latest Windows 10 Insider Preview suggests that S Mode is changing again, and this time it looks like it’s going to be a regular option that can be set at any time. Build 17686 includes a “Switch to S Mode” search item in the Settings app. The actual to enable S Mode isn’t present in this build, so we can’t be entirely certain of how it will work, but this is a strong suggestion that it will now be possible to put a machine into S Mode after the initial installation.
The big question this raises is what will happen to existing non-Store applications once the switch is thrown. There would be a lot of value in being able to install desktop applications and drivers in regular mode and then have those applications continue to work even after a system is switched into S Mode. This would offer the often-essential access to legacy applications while still offering the improved safety that S Mode boasts; it would still be possible to add new sandboxed applications, but arbitrary unsandboxed software from the Internet would be prohibited.
This is ultimately what S Mode should have been all along. The availability of applications in the Store is improving, but for now there remains large functional gaps that only desktop applications can fill. An S Mode that could coexist with carefully selected desktop software—installed by an IT department, for instance—would make S Mode much more useful to many more people.