Unfortunately, there is no process by which you can remove a redeemed product key from an account or move it to another account. You will need to contact Microsoft Support who are the only ones who will be able to address this request.
How do I remove a Microsoft product key from my Microsoft account?
Sign out from all office apps. Close all Office apps. Go to Credential Management. Delete all credentials under the Windows Credentials group related to Office / Microsoft / Adal.
What happens if you uninstall product key?
This will clear the previously installed product key from the registry so that it can no longer be viewed by product key finder programs like ShowKeyPlus. This will not remove an OEM product key embedded in the UEFI firmware chip.
