I did find a situation where using my 'activator' disk and simply plugging in the old key on a clean install did not work. I didn't write down the error message but it was not particularly informative. I made a clone and did an in-place upgrade instead. Instead of taking 10minutes to complete, it took an entire day. It quickly went from 0% to 45% but really bogged down from there, hanging at 99% for hours before it finished.

The benefit of this method is that most or all of preexisting folders, files and win7 apps remained installed, even Classic Shell. It seems that if you start with a fairly clean win7, this complete transfer might be more attractive than a clean install of ten. I am reading that under win10 there is no Easy Windows Transfer app, but maybe it is possible to do the in-place upgrade all over again in a couple of years (on the win7 image) without risk of losing the w10 entitlement that is already locked in. Thoughts?