Not sure I follow, when you say "reinstall" how is that different than "take it down and enter it during installation".
I'm sure the legal answer is that you must have genuine Microsoft media to install Windows legally, but from a practical standpoint you did already pay for the license (by far the thing Microsoft is most concerned about) and I personally would not lose any sleep over doing such a thing. The real ah heck is the method of obtaining that Windows 8 iso, because downloading it from a torrent hosting website would be illegal, making it from legitimate media that you have in your possession might still be illegal but significantly harder to trace.