It seems that flashing from 8500 to 9100 works! When you have a card based on the ATI reference Design. kmf has posted a flash guide on the 3DCenter.de Forums, here is a short summery of it in english:
You need 2 boot floppys with a basic DOS (no drivers) -> on one of these floppys you put atiflash and your original Radeon BIOS. -> On this floppy you create an autoexec.bat which should look like this: Read the rest @ Rage3D.