We had to archive invoices+servicing documentation for warrantied mobos from the supplier to keep a legal licensing chain.
Apparently the free upgrade was OEM, bound to the hardware. I did not know. Either way, I'm from Europe (EU), and here a software license cannot be exhausted via second hand market, so it stands to reason I can buy one second hand. That this isn't what Microsoft support is told to discuss, suuure (even when I explicitly asked for it, they insisted I had to buy it via them).
One time their support just give me a licence for a newer version of Windows - I've replaced the HDD/SSD, cloned/copied it and it was not activated. I contacted their chat support from that laptop and when they asked me for licence on the sticker I mentioned I'll have to come back in 5 minutes since I'll have to turn off laptop, and take out battery to see the MS sticker/hologram.
Support said "No worries, here's a new activation key".
Can't recall if it was from XP to Win 7, or Win 7 to 10.
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And after buying 2 or 3 licences from another website just like G2A (Win 10 was ~€10 on Instant-Gaming) - a bunch of new computers (even brand new assembled desktops) were automatically activated.