I have a desktop with a Pro Upgrade Key, and a laptop with a Core OEM key (both legitimate, both for Windows 8). From my understanding, using WindowsSetupBox to download 8.1 will download a image specific to the edition of Windows you specify (if you enter a Pro key, it'll download a Pro image not usable with Core keys; something like that). Not sure if this is the case with 8.1, but I recall having problems related to this back on 8 (the 8 install image I had from my Desktop wouldn't start installation on my laptop due to key-mismatch). I currently have a 8.1 RTM WZT disc. From my understanding, installing 8.1 from this, and then updating from WU, will put you in the same state as a clean 8.1 install. Is this true (and not just assumed)? If so, do I even have to worry about trying to get a hold of an updated 8.1 image?