If you are installing a new mobo/cpu, you dont need to reinstall windows. Just install it, it will do some configuration during startup, and when you boot, uninstall your old boards drivers, and install the new ones. The one thing is that a windows license will not work anymore after upgrading a motherboard, because it considers it to be a new system. So you will need a new license. And yes unless you want to make your new purchase windows 10, you will be on 8.1 because the free upgrade has expired.