Just been thinking again.
I am a dual boot fan. The future matters not, as I have more than one Windows 8.1 licence. One of those will be kept intact, without the upgrade option.
But, for those with the single licence, who will still wish to dual boot with Windows 10 and, say, Windows 8.1, what will be there situation. They have exchanged there licence for the legalised Windows 10, which, presumably, will be linked to the computer/motherboard. If they then try to dual boot back into Windows 8, I see a possibility that they will find it no longer activated.??