One doesn't have to be ejected from the flight to receive compensation; there is separate compensation for moving back a cabin. I got $500 once (and 150% EQMs) for being downgraded to Y from a mileage upgrade EWR-SFO. There's a table by distance; I don't have the link.

Because of the miles, I had exchanged something of value for the seat in first, so the rule applied. I've also been downgraded on complimentary upgrades with no consideration, and not even my original coach seat back (they don't play dominoes).