I've now upgraded and rolled back more times as I can count. The bad news: none. The good news: every time I again upgrade after rolling back, I am given a brand new one month grace period to roll back to Windows 7.
Seems that when the digital entitlement is once granted, it's there. Done deal. Seems also that there's no limit in number of roll backs / re-upgrades. A scenario I could think of is that a user upgrades to 10 only to see that a certain device driver is still not available, rolls back to 7 or 8.1 and keeps upgrading / rolling back every now and then until the driver will be available and found by Windows Update.