Here is some of the latest scoop from MS about reactivate win10:
https://support.microsoft.com/en-us/help/20530/windows-10-reactivating-after-hardware-change

I think you may possibly be denied...

Now that the date has passed and Anniversary Update 1607 is available, have you or anyone tried moving Windows 10(upgraded from retail license) to a new machine(or new motherboard)? What's the result?

Downgradable even after July 29 deadline?

This only shows MS is not enforcing the EULA at the moment. But they may do something about it in the future.

My understanding of Slartybart's 1) is the installation of your old version Windows on the new machine.
If you've used the free upgrade offer, you won't be able to install the old version Windows...

About 1)
Isn't your base license not usable after free-upgrading to Windows 10 for 30 days?