From Microsoft:"If you install from the ISO/media, as long as you start the upgrade from within a Genuine/activated copy of Windows 7/8.1 (e.g. by double-clicking Setup.exe), you shouldn't need your CD key.However, doing a "clean install" (for example, booting from the ISO/media, rather than launching setup.exe from within the existing Windows installation), you will need to enter your CD key again.
Why the rush to Windows 10?
I'm having this issue with activation. Lucklily I'm doing this on a spare computer for testing, but still...I started the process within Windows 8.1 and followed all the prompts. Now, it says no activation code and the one that really is on my machine isn't working for some reason (though 100% legit and bought within the last month).
I saw some confusion about whether this would work with an upgrade or OEM vendor installed license. My tech spidey senses tell me the safest way to go about it would be to do the regulated upgrade per GWX, log the current and potentially new key and then do a clean install.