Installing windows 10 in a clean petition

[Windows Central Question]

In short, if I use my win 7 key to activate win 10 in a petition, will my oem ver of win 7 still be active or will windows 10 take over as the main active windows ? And If I do activate windows 10, I take it there?s no way of reactivating my original windows 7 key if I uninstall windows 10 ?