Because the digital entitlement is locked to your motherboard.
As to log in to get the full W10 download, I've never had to do that. I just google Microsoft Media Creation Kit and I'm done.
Since the license systems changed my bet is you'll have to buy a new W10 license. Unless there is more story. Like your old W7 was W7 Full version Professional which had migration rights. But the old key or digital license for W10 is in this case dead. You will be calling Microsoft to tell them your old motherboard died and you are repairing your PC. Even if you are changing the motherboard for other reasons.
If you let on that this is not a repair, they sell you a new license. Now you know.
I am planning to upgrade my hardware. I have the full version of Windows 10 Home which I downloaded when it was free at the release. Im wondering if I change my hardware and then try to boot with an USB with windows on, can i log into my Microsoft account from there and download the full version of Windows again, since i have the digital license?

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