Yes, you can upgrade from Windows 8.1 to a new motherboard.
Yes, you can upgrade from Windows 8.1 to a new motherboard.
Your Dell system already had Windows 7 and an 8.1 license installed. You've upgraded to Windows 8.1, but you're concerned about the proprietary motherboard and its compatibility. If you decide to change the motherboard, you should be able to keep your Windows 7 license with it. However, ensure the new motherboard supports your current system requirements.