With your original motherboard NOT with a different board. You do not need a key to install win10. You need a key to activate win10 for the first time. Each install after that will auto activate as long as you do not change hardware.
If you are upgrading a dell, hp, etc; you need a new key.
Is this a big box case you are attempting to use ie was this a hp, dell, etc? If so, you may need a new case as well. Dell, hp, etc stuff is proprietary that is it is made to the mfg's standards NOT industry standards. A standard atx motherboard may or may not fit a dell case; pci slots may or may not line up, pw supply may or may not work.