This is precisely what happened. I was booked in an aisle seat second row from the front. On check in the agent moved me to an exit row seat, at my request. When my boarding pass was scanned it beeped but I knew it wasn't a 'good beep' as the flight wasn't full. I was changed to a middle seat 5 or 6 rows back. Luckily I was able to move to another aisle seat once every one was on board.

I asked what would happen on full flights or overbooked flights. The purser stated that in these instances customers could either opt to pay the extra for the exit seats or they would be placed on a different flight and the seats would remain empty. This is very hard to believe but she was adamant that this was the new rule.