The information here may be helpful:
Downloading Windows 8.1 using a Windows 8.0 Product Key - Windows 8 - Software & Operating Systems - Dell Community
I have wrote a means to download Windows 8.1 with a Windows 8.0 Upgrade Product Key.
In addition I have wrote a means to clean install Windows 8.1 using a generic Windows 8.1 install key and to activate Windows 8.1 using your actual Windows 8.0 key.
Note I am linking to the Dell Community which links to WordPress because last time I linked to my guides I got banned for blogspam.
As I don't have a Windows 8.0 system with OEM SLP. I would be interested in knowing whether the last step works. Install Windows 8.1 using the generic install key and then once when Windows 8.1 is installed remove all product keys using slmgr.vbr /upk and then restart your computer to see if the 8.1 install picks up the 8.0 key within the system BIOS?