Today, two of my Windows-10 eligible machines have told me they can't yet update to Windows 10, reason given is that the Manufacturer (Nvidia!) "hasn't released a driver for Windows 10".
So at least it won't be breaking lots of computers today. not with duff Nvidia drivers, anyway.
Just about to back up the hard disk, swap for a different spare graphics card, and see if the Windows 10 upgrade will run.
Ian