i think it's not a good thing to do although you may try it.
i have a friend of mine who has a voodoo 3 3000 AGP and went on to upgrade his video bios. he called me up late at night and told me that his video card has a problem and its about the flashing thing. his procedure was wrong and he flashed his default bios (AGP) with a PCI version. although no major damage was caused, i managed to restore the original bios and it worked again.
PS. this totally convinced him not to flash his bios again.