I upgraded from W7 Pro to W10 Pro a few days after the upgrade came out. It worked, though I immediately turned the PC off on seeing it completed.
On sign-in the next day the OS was unusable (system hangs and complete freezing almost instantly) so after numerous restarts and attempted troubleshooting with MSCONFIG, I rolled back to trusty Win7.
I always run into these sorts of issues, and that is why I prefer to clean install every time, but I had to upgrade so it would recognise my valid Win7 key and give me the goods.
Now, my question is, even though W10 was only on my system for about an hour (and 90% of that time locked up) will my system's hardware signature be registered with Microsoft as 'activated'? Will I be able to fresh install a Win10 ISO and have it activate happily, or will I have to format and clean install Windows 7 and then upgrade to Windows 10?