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In the first year of free upgrades, this was not an issue as you simply reused the key on new device (wiping windows from old device of course).
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When Windows 10 first came out, people said - hang on, I had a 7/8 retail key and if I upgrade, my key has been converted to OEM status in effect which is a downgrade in status. HOWEVER, Windows 10 does not really use the product keys anymore other than just to verify you have a legitimate licence for that device, and if so, you get granted a digital licence tied to that hardware and you only get a placeholder generic key of no use. Windows 7 Home was oem, then an upgrade to 10 Home is LEGALLY OEM as well. The EULA states clearly an upgrade inherits the same status as the original key. OEM keys are according to the EULA non transferable to new devices. you can simply activate new pc/mobo be entering the key at the "Change Product Key" screen (but if transferred too many time, it may get temporarily blocked, requiring telephone activation. The EULA definitely states retail standalone licences can be transferred to new PCs i.e.
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due to a hardware change (or some unknown glitch).
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The digital licence link to the MS account is a secondary protection mechanism that allows you to MANUALLY reactivate Windows (using the activation troubleshooter) in the event it fails to do so e.g. First and foremost, digital licences are tied to the hardware, and if you reinstall OS on same PC, it will activated regardless of whether a local or MS account.
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