Thanks shawnfuller. So:
a) You have it on a clean w10,
b) I am OK on Lenovo upgraded W7>W10 2015, others as well.
So apparently not directly due to W7 upgraded PCs?
BUT- as briefly in my previous post, licence on my problem machine has "a history". Originally purchased online 2017 to use to upgrade my then Dell , 2009 XP, upgraded to W7 2014 on original licence.. Had several abortive attempts at W10, failed other reasons. Eventually installed W10 OK , but would not activate against that licence. But eventually activated itself on original XP>W7>W10. So that key not used, should be available.
Current PC purchased with trial W10, aimed to use it there. Again, assorted issues with that. Eventually had a remarkably constructive telecon with MSoft - agreed I effectively "owned" an unused licence, but "corrupt". Issued me a new key. PC activated OK. But now shows not that licence key - but "Digital". AOK until now.
Conceivable KB5016616 issue is some deeply buried licence or security bug?
Doesn't seem that anyone on this thread with a failure has actually found a solution? Including some clearly very experienced people.