For many years after the official end of the free upgrade program allowing machines running Windows 7 or 8/8.1 to upgrade to Windows 10 without paying for an additional licence, it was still possible to upgrade using an ISO file or USB installer. This used the previous licence to activate Windows 10 and all was well.
As of 20th September 2023, Microsoft have stopped this method from working, meaning that free 7/8/8.1 to 10 upgrades are no longer possible, as Windows 10 will not activate using the old licence.
However, you can still perform the upgrade and, as long as you’re willing to put up with the message about activation in the bottom right of the desktop and an inability to do some minor things such as set your own background, Windows 10 will work perfectly unactivated.
Upgrades from Windows 10 to 11 still remain free of charge at this stage as long as the hardware meets the requirements.