Upgrade keys only work when you are upgrading from an earlier version of Windows.
Bear in mind that you need to install the W8 upgrade version OVER the already installed older-version. A clean install will allow you to install W8-upgrade-version but won't let you activate it properly (even if you have legit keys of previous versions - it just doesn't work that way).
However, you can do a clean install of W8-upgrade if you want and after everything is installed you need to do a registry tweak to allow your W8-upgrade-key to be recognised alright. Just google for it.
I had a similar problem. I had W7 (legitimate) and I bought an SSD and W8 upgrade.
I decided to do a clean install of W8 and had my other hdd's unplugged. After W8 was installed it wouldn't recognise my product key and it wouldn't let me use my W7 key to prove I was legitimately upgrading. So I did the registry tweak (super simply) and it then accepted my key as normal. A bit annoying they have it set up like that forcing you to install it over the previous windows instead of allowing clean installs for people that have legitimate older-win versions.
You could always try the key on your existing version and see if it works, if not, install the W8 upgrade version and activate it as I suggested. Of course, to be all above board you should have an older paid-for version of Windows which you can theoritically upgrade from, which I guess you do, right? right?
Not sure if the upgrade W8 will allow you to install over an existing W8 version or whether you'll need the clean install, but the activation can be done with a clean install as I described anyway if you end up doing that.
