A little late to this thread but.....
Last year I bought a small, light Lenovo "netbook" thing that came with Windows "S". I had previously researched and found that on activation, you could indeed "upgrade" to the full-blown Windows 10 operating system,, but not revert back. I dutifully upgraded with no issues and no regrets. Like others, I fail to see why any normal "domestic" user would use "S" and cannot see why retailers make such a "fuss" of stating that this is the operating system but not qualifying it by saying that a free alternative to "full-blown" 10 is available