2960xm cpu upgrade - microcode failure - m6600

...trying to upgrade CPU on a M6600 from i7 dual core to 2960xm quad, but it won't boot - it sends up 'microcode update failed', and a system check shows it as running at a slower clock speed

- which BIOS version is recommended (I'm currently running A15)

- will a fresh system reinstall be required?

...any suggestions or input anyone might have would be greatly appreciated