I had saved one Windows 7 Professional N product key to test how well Digital Entitlement works in case of rolling back to Windows 7 after upgrade, then upgrading again after the free offer is gone.
Upgrading that machine as we speak, will then roll back to 7 before tomorrow night and try to go back to 10 sometime next week. Not a big deal if I lose the key but it should work.
If it works I intend to test if it works continuously, upgrading and rolling back as long as I can. I am doing it on a Hyper-V vm, it will be easy to roll back and upgrade daily.