First you need to extract the GPU BIOS and keep a back up file, I use atiwinflash but its a windows application, dunno in linux.
Then with a BIOS editor, you change straps and save save as name.rom, again the ones I use are windows base and not linux.
Once you are done you need to flash the new modified BIOS into the GPU and reboot.
Make sure you save the original bios tho