Today marks my first failed attempt to upgrade an activated Windows 8.1 Pro system to Windows 10 Pro. Microsoft recently announced that free upgrade paths from older versions of Windows would end with older software keys no longer activating Windows 10 installations. I tested this today with a VirtualBox running Windows 8.1 Pro that I upgraded to Windows 10 and can confirm that it will not activate. At this time upgrading from Windows 10 to 11 should still be possible and free.