Hi,
I decided to upgrade my Windows 7 to Windows 10. I have Witcher 3 installed without GOG Galaxy. I found the savegame files in C:\Users\Username\Documents\The Witcher 3\ So I save these somewhere safe, upgrade to Windows 10, install Witcher 3 and then just copy these files somewhere? (And a maybe silly question: what happens if I don't uninstall Witcher 3 before the upgrade? Is there any chance Windows 10 would take it over, and it would work, and my savefiles would be there etc., or is that too much to expect from Windows 10?)
Cheers

Should be okay,but to be safe I would save first on to an external drive or usb.

Your saved games will still be in the same place.
When I went from Win7 to win10 the saves were there, but I couldn't access them.
The operating system had taken ownership of the C:\Users\Username\Documents\The Witcher 3 folder.
Once I right clicked on the properties > security and made myself the owner of that folder, they worked.
Then a Windows update last November once again took ownership of that and a few other folders, but no problems since them.
With the continued updating of Win 10, you may have no issues at all now.