I thought that substitution of a smaller aircraft is one of the exceptions in which IDB compensation is not legally due:

Yes - that is the source of my confusion. Twice in the last 10 years I have had a ticket for a flight and after a substitution of a smalller plane occurred was denied boarding because of space consideration. In one case the delay in re booking was over 6 hours. I was informed no compensation was due in either case.

Is the rule different for international flights?