BIOS versions are not for support of OS versions. BIOS directly relates to hardware support. In that some hardware supports particular features of a particular OS, like GPT partitioning and secure boot, but any newer version of the BIOS should have all support that was included in prior versions of the bios, so upgrading won't diminish that support at all. If you are looking for support for a newer feature to support a feature of Windows 8.1, like UEFI hardware or GPT partitioning, and your motherboard is old enough, there may simply not be a newer revision available that has the features you are looking for.