I upgraded my only Windows 8 install: the one in a virtual machine. FWIW, Windows 8.1 has a new kernel version number, 6.3 (up from 6.2). Any software doing OS checks by kernel version will need to take that into account (guilty!).
They did that back when Server 2008 R2 was released (confusing as anything, as Server 2008 was 6.1 - same as Vista - but Server 2008 R2 was 6.2 which was the same as
Windows 7!)
Anyway, similar experience to Terry and dismal UX fail on Microsoft's behalf. Their site had links to "upgrade to 8.1" but clicking it launched the Windows Store where nothing was visible. I had presumed I was up to date with the Windows Updates for that Windows 8 install, I Googled around for what felt like an hour (probably closer to 5 minutes, but I can see it taking Joe Somebody a day or two

for an offline installer (I actually already had the 8.1 ISO from MSDN) or a direct link to the update, but could find nothing. Running Windows Update indicated several "updates for Windows 8" and many security patches (even though I was up to date just two weeks prior!) but no "Windows 8.1" option.
I chose to install all available Windows Updates, still nothing. Click the links again, still nothing. Then I figured I'd run Windows Update again, and lo and behold, no Windows 8.1 listed, but several more generic "Update for Windows 8" KB entries/hotfixes/whatever. Installed
those, ran WU again, found more updates, installed, and WU kept finding the same update over and over and over again for me to install and install it would, just to keep finding it available again and again. Launched the Windows Store desktop tile (no way to get there without Metro?) and suddenly I could see the fullpage tile everyone was talking about that said "Upgrade to Windows 8.1". Upgrade took forever, I went to the gym, came back, and it was stuck on a prompt midway through (something silly like "are you sure you want to do X?") meaning I had to find something else to do until the upgrade finished. But in the end, it did go through.
I can't comment on whether the shell was reset as it was quite-literally a stock install with no personalizations or customizations. Took some fiddling and a few false starts to "disable" Metro on my own (instead of following a guide) — I honestly can't see why these Metro options are available by right-clicking the
taskbar and changing its properties. Oh well.
Yet, I'll probably still upgrade my desktop to Windows 8(.1), now. I'm still running Windows 7 and having no real problems (other than the usual Windows stuff) and really don't have the time to format and reinstall (I have absolutely zero confidence I can actually utilise the "upgrade" feature on my real dev machine, it absolutely will need to be clean-installed)... Some of the new APIs available in Windows 8 and the new task manager and other small enhancements here and there are just nagging at me once-a-week or so... we'll see what happens!