I had been having some issues with activating a recent install of Windows 10, but I heard about an update which was just over the horizon and decided to wait and see if it fixed my issues. I recently checked for updates and to my pleasant surprise found the update awaiting download and install.
When the installation was complete Windows reassuringly told me that "all my files were exactly where I left them", this was true but that was about all that was exactly where I left it: I use a dual GPU Nvidia graphics card and triple monitors which use quite a few non-default settings to work nicely, including a few registry hacks to get colours to show accurately. When I booted into the update all settings were at their defaults (or strange other values that nobody in their right mind would set) and only 2 of my displays were working, both showing weird colours. It was a nice touch that Windows appeared to have detected that my desktop was a little too well organised; with items grouped in nice rows depending on usage cases (I'm OCD like that), and graciously took the time to clump them to the far right of my screen. The update also changed my colour preferences, changed my language (US English instead of Australian English) and reset all default taskbar items. It has also helpfully reinstalled all default apps as it doesn't seem to understand that I already have Skype, and that I couldn't be less interested in getting Microsoft Office.
Edit: Seems my keyboard driver (Logitech G510) also bit the dust and had to be reinstalled.
I was originally glad to see that my activation problem had resolved itself, but after several restarts to apply registry tweaks to get my monitors displaying the right colour it promptly deactivated itself again and as an added bonus Windows now takes about 4x as long as before to start up (about 15-20 seconds from Windows logo to login screen as opposed to roughly 4-5) when booting from a fast SSD.
I am currently in the process of freshly reinstalling a previous update of Windows 10 and attempting to upgrade before installing any drivers because I hope this might fix the insanely long boot times as I believe this may be a driver issue. Hopefully most of these problems can be fixed and I'm hoping someone has some information about how to upgrade safely next time.