Windows 8.1 is essentially a free large update package for Windows 8, not real upgrade. So all Product Keys and Activation Keys are also working to the corresponding editions of Windows 8.1.
Below, AppNee searched through the whole Internet and screened out 7 Product Keys which are universal for all editions of Windows 8/8.1 (including all languages, both 32-bit and 64-bit); such as Retail (Full Packaged Product – FPP), OEM (Original Equipment Manufacturer), Upgrade, RTM (Release To Manufacturing), VOL (Volume Licensing for Organizations), MSDN, TechNet, Preview...
Windows 8.1 Preview
NTTX3-RV7VB-T7X7F-WQYYY-9Y92F
4R6KN-TWVVD-JGD3D-HQ8HY-VQVQQ
Windows 8.1 (Core | Multiple Editions) 334NH-RXG76-64THK-C7CKG-D3VPT
Windows 8.1 Pro XHQ8N-C3MCJ-RQXB6-WCHYG-C9WKB
Windows 8.1 Enterprise MHF9N-XY6XB-WVXMC-BTDCT-MKKG7
Windows 8.1 Single Language
Y9NXP-XT8MV-PT9TG-97CT3-9D6TC
TNH8J-KG84C-TRMG4-FFD7J-VH4WX (CN)
Windows Server 2012 R2 Standard Server
D2N9P-3P6X9-2R39C-7RTCD-MDVJX
NB4WH-BBBYV-3MPPC-9RCMV-46XCB
Windows Server 2012 R2 Datacenter
W3GGN-FT8W3-Y4M27-J84CP-Q3VJ9
BH9T4-4N7CW-67J3M-64J36-WW98Y
Windows Server 2012 R2 Essentials KNC87-3J2TX-XB4WP-VCPJV-M4FWM