Ron wrote:
If I upgrade from Win8.1 to Win10 and use it for 28 days and then
rollback to 8.1 can I upgrade to Win10 before the 1 year free upgrade
period is up?

Ah yes, hair splitting. We could have fun with hair splitting,
if we had clear policies to go on. But there aren't any.

Obviously, the free upgrade offer ends Aug.1, 2016. We can count
on that. There's no particular reason for Microsoft to leave the
qualifying OS and Win10 both in genuine state. But as for how
they figure out what you're using, I haven't seen any suggestion
of whether sampling will be used, only a check on Aug.1, 2016,
or what they will do.

I can't promise anything, but I took the Win10 DVD (from
MediaCreationTool) and loaded it into VirtualBox, and
the "skip" button appeared. I was able to do
an installation of Win10 with no license key. But
the OS was not activated at the end (as you would
expect), which means I could test that OS for
30 days.

What I don't know, is whether the OS "sensed" something of
the setup, to be presenting the "skip" button. Really,
the skip button should not have appeared, but it did anyway.
I've attempted to do other tests, like install on real
hardware, and it won't give the skip button, and it
just sits there asking for the key.

So if you want to play with it, load it into VirtualBox,
turn on the experimental 3D support features, and see what
happens. You might get an evaluation out of it.

Paul