Product Key can now be a “Digital License”. This seems to be true for upgraded Windows 7 keys.
To prevent activation issues after hardware changes: link MS account to digital license asap (Settings > Accounts).

View current license:

slmgr.vbs /dlv
wmic path softwarelicensingservice get OA3xOriginalProductKey

powershell "(Get-WmiObject -query 'select * from SoftwareLicensingService').OA3xOriginalProductKey"

Update product key:

run slui.exe or goto Settings > Activation

MS Activation servers:

activation-v2.sls.microsoft.com
validation-v2.sls.microsoft.com

More info:

https://support.microsoft.com/en-us/help/20530/windows-10-reactivating-after-hardware-change