Actually the fare paid seems to be irrelevant to the compensation, from what I've heard (I've never been invol downgraded on CX). At most airports you are given an envelope at the gate with the fare difference between published one-way J and Y or F and J, in cash. Doubt it happens more than a few times per year, certainly nothing like bumps on some other carriers. Usually when it happens it is due to DM exercising their purchase rights for a sold-out flight.