When you upgrade hardware that causes windows needing to reactivate, you do not not need to reenter the product key again.

If you cannot activate windows you can start Run and type in SLUI 3. This will start windows activation. Key in your product key and activate. If cannot activate you can hoose to activate manually and call microsoft to get activation code.

Your current Windows 7,8,8.1 key CAN BE USED TO REINSTALL WINDOWS 10. This one 100% confirm as i have done it many times already.

Requirement for this is THAT THE PC HAS UPGRADED WINDOWS 10 AT LEAST ONCE from old windows version.