Note that it is version-specific. I just flattened my Surface Pro 2 and used a non-OEM version of Windows, which required its own product key. Also, Windows 8 and 8.1 have different product keys, but if you upgrade OTA to 8.1 it will install the same SKU (OEM) for 8.1. Changing the OS is complicated with UEFI and secure boot, the usability is insanely bad, with a lot of boundaries being blurred. It's the worst experience I have ever had when installing a clean OS.