The voodoo driver is generic to the entire voodoo range (at least, the mediator compatible ones). The only thing you need to change is the voodoomem envarc setting. The rtg graphics address remains the same as does the P96 monitor driver and settings.
Just run meidatorconfig before you change the boards and select voodoo3 as your GFX board - the program will change the settings for you, then just switch off, change boards, reboot.
Or do it manually through setenv envarc:mediator/voodoomem (although can't remember whether it would go to 8mb or 16mb - think it depends whether mmu is supported or not, which with BPC it isn't). Probably safer to use the mediatorconfig program if not certain.