Get the Windows key from a pre-built PC setup.
Get the Windows key from a pre-built PC setup.
I understand your setup and the situation. The Windows installation should work fine on a new SSD, especially since you're using retail parts. For the Windows key, contacting Microsoft support is a good idea—they can help reset or reactivate it. Regarding the upgrade, since you're moving from 8.1 to Win10 and the free upgrade period is over, you should be able to upgrade again for free if you contact Microsoft directly.
You can set it up on an SSD and it should function fine, even after replacing the motherboard and CPU. It treats the system as a fresh setup, so Windows 10 won’t be available for free anymore.
Enter the required key during installation. If it doesn’t work, contact support and provide an external number to receive a replacement code.
Be mindful that certain keys for ready-made machines can't be moved, so keep this in mind when contacting Microsoft.