I had the same problem with the mismatch bios in DOS.
To revert the old bios you must:
type in C:\ ATIFLASH -UNLOCKROM 0
it will say ok
C:\ ATIFLASH -F -P 0 (BIOS NAME).BIN
And everything will be just fine with the new(old)bios . No more mismatch SSID.
The problem was that...