I have Win10 Pro x64. I had windows 7 and I wanted to do a clean install rather than doing the upgrade from within windows (I dont trust Microsoft with a clean upgrade). I contacted Microsoft helpdesk inquiring if I could use a Windows 10 ISO and use Windows 7 key (or maybe trade in windows 7 key for a windows 10 key) and the answer was no.
So I tried downloading windows 10 ISO and tried it any way (by booting into the windows 10 setup) using Windows 7 Key and it worked. Formatted my HDD and a clean install. No issues whatsoever. I would rate Windows 10 higher than Windows 7 in day to day use.
If your PC has a UEFI BIOS (most laptops with Windows 8 and above), the BIOS saves the key and the Windows 10 setup is smart enough to get the key from there and it wouldn't prompt the user for the key. Still, I would recommend noting down the key before the start of the setup. Another good thing is that in DVD Boot mode, the setup asks for key upfront so you know if the key is accepted or not. I am not sure though if there could be issues with the activation later. I haven't run into any issues.
All above is based on my experience with 4 PC installs of Windows 10 in different configs. Good luck @@LaRSin and let me know if I can help with anything.