How to make my computer forget the key for my Windows 8.1 upgrade?

I have a W7pro machine.
Bought a W8.1 upgrade. (Yes, I have good reasons to do so, don't flame me on that one.)

Turned out it wouldn't keep my software installed. I didn't want to reinstall everything (a three day job), so I called MS, and after two hours of blundering finally they downgraded my W8.1 key to a W8 key, and I should install W8, which should let me keep my "apps" (software) installed, after which I can upgrade to 8.1.

HOWEVER: the 8.1 key is somewhere in my system, and I cannot download or install W8: it will always attempt to install 8.1.

I've tried:
"Download windows 8" (below the "download Windows 8.1" button), downloaded it from the W8 sales link, downloaded W8 on a different computer and transferred the download... it doesn't matter. I cannot upgrade to 8, only to 8.1, I guess because the 8.1 key is stored somewhere. The reason I think so is because the upgrade process doesn't let me enter a key. (And after some trouble I get the oppurtunity to enter the key, it just says "invalid key" because it is trying to upgrade to 8.1 and W8 keys aren't valid).

Frustration!

Anyone any idea how to get the 8.1 upgrade key out of my W7 system?

Answer

Thanks everyone. I solved it myself by finding and removing the installation files. It is a while ago, so I am not 100% sure of which folders I removed. It is very hard to find when Googling :( . I think these are the folders I removed, though I repeat: I am not 100% sure.

Anyone with the same issues, try this:

Remove the folders

C:\Users\USERNAME\AppData\Local\Microsoft\WebSetup 
C:\ESD\Windows

Of course, replace USERNAME with your user name.

Once I had the folders removed, installation went (almost) perfectly.