No one knows for sure.
I suspect any preview versions will be upgraded to the final version using Windows Update (as the preferred new delivery mechanism for all future OS upgrades).
But as that'll leave all the windows.old etc cruft on my test machines, I'll just do a fresh install. Assuming the licencing doesn't screw me over.
Well, they're also happy for counterfeit XP installs to upgrade to 10.
XP will not upgrade to WIN10. You need to do a clean install.
A non genuine Win7 or 8.1 install will though. The resulting WIN10 install is still marked as non genuine and will have the usual limitations as such.