I would be hesitant too. 40 days is possible but certainly not healthy.i know it's a Black Australorp hen and it's her first time to become broody so i'm a bit hesitant to switch the eggs...I've been through a lot of Broody hens before and this is the first time that none of the eggs are fertile and i don't want to waste the broody hens time or try to break her out of her broodiness...Thanks for your advise...
Are you sure the eggs didn’t start? I have a torch that shows blood vessels in white eggs, but not in light brown eggs.
So depending on the egg color, the shell thickness, and the quality of the torch I would decide to discard them or try to hatch these. In doubt I would let the hen sit for another 2 weeks (max).
If I wanted to start with new eggs, I would prefer starting later/ with another broody.