I'm not 100% if the Windows 8.1 key will be disabled as a result of the upgrade. In theory if you wanted to dual boot, you will need to first get your Windows 10 key with magic jellybean keyfinder. Then you would need to install Windows 8.1. After that is installed you would need to shrink your Windows 8.1 partition. Then install Windows 10 and select Advanced mode and select the un-allocated space on the drive. The golden rule for installing multiple versions of windows is always install the oldest first.