HI Manu. I'm Greg, 10 years awarded Windows MVP, here to help you.
The issue is the hardware motherboard change, after which only a retail license can continue. If your Windows 10 was converted to a Digital License then you may be able to sign into the new hardware with the same account and run the Activation Troubeshooter to transfer the license. This is explained in this Microsoft document:
https://support.microsoft.com/en-us/windows/rea...
If not then it requires contacting Support to move the retail license, which it will always do providing you still have the key. If you bought the license from Microsoft it may be in your Order history here:
https://support.microsoft.com/en-us/help/402731...
It is possible that if you still have the install that originally had the retail key installed, it may still be stored in Activation Settings. To try to find it run ProducKey which is from the respected software maker Nirsoft so ignore any claims it is a hacker tool, that's what most keyfinders are classed as:
https://www.nirsoft.net/utils/product_cd_key_vi... and observe closely all keys. It will NOT be the OEM-BIOS key or Generic Version key shown here:
https://winaero.com/windows-11-generic-keys-for...
Any other key may be the retail Product Key which you can try to give to Support.
Report back results for each step so I know what else to suggest.
Feel free to ask back any questions. Based on the results you post back I may have other suggestions if necessary.
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