Thank you for the last comments
Yes, I'll definitely change them.
I still don't really understand why they can stay forever on the flywheel without being changed if you don't touch them, but once they have been unscrewed, they are likely to brake (?) so need to be changed. I guess removing them stresses the metal some how, but...
So, isn't it the same with the rear axle bolts? It should be, because to me they get the same shear force as the flywheel bolts, no?