Thanks for sharing!
Will try when I get home from the office.
There is a Pro key and a Core key? The one you use to install has to match the 8 key your going to activate with.
For me that trick worked just fine, and I was a current Win 8 owner. The trick was to you need to start the Win 8 download first. Once the download has started you cancel the download, then you click on the install 8.1 and it now starts downloading the 8.1 ISO.
Regarding the "generic" key, I found on the internet, that we can't use our Win 8 key for a clean install of Win 8.1. So for the clean install you need an "installation key" which you can obtain on the MS web-site. I copied my ISO onto a USB stick, started the installation, entered the generic key, finished the installation without a problem, then activated with my Win 8 product key without a problem.
do an internet search for Windows 8.1 generic key and you should get enough hits listing keys you need for the installation