The question about Retail licenses and Windows 10 is one Microsoft has not answered. It would seem logical if you are using Windows 10 after upgrading using that license, you will not be able to use it again unless you stop using Windows 10.
The question about moving an OS to another system, since yours is not an OEM/single computer type install, is relevant. Personally, I am going to assume the 8.1 license is valid as long as there is no current Windows 10 install present.