Part of the problem may be how the psu is wired for that imac, there is no soft power on, also it dosen't have and speakers, and seems it won't give the startup tones through the headphone socket, so it makes this kinda hard but what i did was:

run the updater, wait for the imac to power down

pull the power

hold the programmers button and plug the power back in

at which point it would sit at a white screen for a bit then reboot

if i kept holding the button it would go to open firmware, if i let go it would just boot normally and prompt the update failed.

i also tried not holding the button, and got the exact same result, and i tried without pulling the power after it shuts down, and got the same resulat

Who knows what made it work, maybe no one ever bothered trying a older version of the os, but i find that hard to believe... i did try a differen't cpu card, bur as i didn't remove the updater from startup items it could have done something to the firmware...

The only way to ever really know is if somone else trys it

it would be intesting to see if it is possible to re enable floppy support in os9, but i have no idea where to even start on that