This is conjecture so no one take this as reliable information please; but I thought that Bios should be and was OS independant because it is there to control attached hardware. It does however communicate with the OS suppllying info about the hardware.
The thing that makes me query the OS independance of a BIOS update is the fact that the OS is often specified with a BIOS update.
I just do not want to take a chance of flashing the BIOS and rendering the PC essentially useless beyond my ability to repair.
If I do not upgrade the CPU this computers days are numbered and I do not want to put it out to pasture just yet.