I've done the Windows 10 upgrade on 3 of my systems so far. I haven't upgraded my main system yet because I can't be inconvenienced. I haven't had any real problems with my upgrades, but I will say that I don't really believe in OS "upgrades", so all of my upgrades have been done with the intent of immediately following them up with a partition dump, re-partitioning, formatting of the partitions, and finally a new OS installation on an empty drive.
Since you are not given a Windows 10 Product Key as part of your free upgrade, I did quite a bit of reading to find out how we would be able to do a clean install without the Product Key. In short, you have to go through some extra steps or it won't work. Unfortunately you can't just simply download the Windows 10 image, put it on a DVD/USB, and then reboot and install.
What I found through my online searching was that you absolutely must perform an OS upgrade first - from a qualified version of Windows that is already activated. So if you're upgrading from Windows 7, you must make sure that that version of Windows 7 you are upgrading from is fully updated and is itself properly activated. Then you have to upgrade it to Windows 10. The upgrade process will not require you to enter a Product Key. Once it has been updated, Windows 10 will eventually activate itself on that system. Do not attempt to do a format / clean re-install until you have checked, double checked, and triple checked that Windows 10 is showing that it has been activated.
Apparently when that activation happens, Microsoft essentially remembers that that hardware has been activated. And according to what I read online, Microsoft will still acknowledge that that system is activated even if you change out some hardware. If you change out too much hardware, you may have to go through an additional re-activation process at some point. I've had to do that in the past and it isn't difficult. But if I were planning to upgrade the hardware on my computer, I would do it prior to upgrading.
Getting back on track...
Once the system has been properly upgraded and activated, you can then completely wipe the system clean, reinstall Windows 10 onto a freshly formatted drive, and once that is done, it will still be activated. (Note that when you install Windows 10 after the drive has been formatted, it will ask you for the Product Key 2 different times. Just "skip" it in both places.)
This process does work as I've done it on 3 different computers now.
The first time I did it I did experience a problem only because I didn't follow the proper steps. I upgraded a Windows 7 computer to Windows 10, and then immediately formatted and did a clean install of Windows 10 on that system.
The problem was after initially upgrading to Windows 10, I didn't wait for Window 10 to have a chance to activate. (Only because I assumed it was essentially instantaneous.) So when I did the format/reinstall, the system never showed that it was activated due to the fact that the upgrade process was done improperly.
This cost me a lot of time as I had to then go back and do a format/reinstall of the original Windows 7. And then activate it. And then do the Windows 10 upgrade (a second time on that system), and then wait for it to activate. And then finally do a Windows 10 clean install (for the second time on that system), and then it was properly activated. Due to that first botched attempt though, I didn't have any problems on the next 2 systems as I then knew to check (and double check) that Windows 10 showed that it was activated prior to doing the format / clean install.
I'm not content to upgrade an entire operating system from one major version to the next. In my experience, an operating system can't even stay de-cluttered on its own, let alone when you install a new operating system over top of an old one. I think a clean format/reinstall is always ultimately easier and less problematic than trying to upgrade. But that isn't going to be true for everyone I suppose. I keep all my "data" (music, photos, downloads, documents, pdf's, etc...) on a different physical drive. (And I have a mirror of that drive for backup.) So when it comes to doing a clean install, I am only inconvenienced by having to reinstall all my programs. Which is definitely inconvenient ... but is (to me) less inconvenient than trying to deal with a heavily polluted operating system that is a hodgepodge hybrid of an old OS mixed with a new one.
