Happened to me a few years back as well... Gate agent had to manually re-seat the whole plane as the auto switch really did a number on everything. The redcoats explanation was that when certain equipment swaps happen it completely locks seat requests for almost the entire aircraft from anyone besides gate agents, and it happens relatively infrequently so not many agents know the override. My particular flight was overbooked by 10, which is my guess as to why the swap happened.