Take a look at those posts for more info.

RETAIL KEY ONLY.
Windows 8 and Windows 8.1 ISO - Download or Create

How to download Windows 8.1 ISO with a Windows 8 key.
How to download the Windows 8.1 ISO

Activate retail Windows 8.1 with Windows 8 Product key.

You can not use the key extracted from the UEFI/BIOS firmware or a OEM key to download the Windows 8.1 ISO, you can only use the key to ACTIVATE.

The generic keys can only be used to install Windows 8.1 & not ACTIVATE or to download the Windows 8.1 ISO .