Hi,
Do you mean you want to replace your hard disk on your Existing Lenovo PC which running Windows 10 with your old one which run Windows 7?
As Windows 10 will associate your hardware information with your activation information on Server side, so if you replaced your hard disk, your Windows 10 activation will fail and need to re-activate again.
If you want to add hard disk to your Levono PC with the older disk, we could refer to the link below:
https://helpdesk.originpc.com/support/solutions/articles/9000124011-how-to-add-a-hard-drive-to-windows-10-
Note: This is a third-party link and we do not have any guarantees on this website. And Microsoft does not make any guarantees about the content.
If I misunderstand your issue, please reply to us directly.
Bests,
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