Originally Posted by IluvSQ

To answer the OP's original question, yes, it has happened.
My wife and I were seated in J ( one paid, one reward) on a YYZ-PVG AC
flight when it was cancelled for mechanical reasons.

We were put in Y on an AC YYZ-NRT flight, then again in Y on a NRT-PVG
flight. 2 downgrades in one day ( although the second was technically the
next day since we crossed the dateline)