Activation in Windows 10 - Windows Help
> Activating Windows 10 (Version 1511 or higher) using a Windows 7, Windows 8, or Windows 8.1 product key
During the free upgrade, you can use a valid Windows 7, Windows 8, or Windows 8.1 product key to activate Windows 10[ (Version 1511 or higher).
> Activating Windows 10 after a hardware configuration change
If you upgraded to Windows 10 using the free upgrade offer and then made significant hardware changes to your device, such as replacing the motherboard, Windows 10 might no longer be activated. For help on how to activate your device, contact customer support.
It's probably just the wording of the above, always referencing the Free Upgrade, that's bothering me.
If you can use a previous Qualifying license after 29 July, that would essentially mean the Free Upgrade doesn't end. I suspect that the servers will only accept Win10 keys after 29 July.
The only thing I can imagine is a cross reference to an existing digital entitlement based on the key during a phone activation.