That happens all the time. That's why Flyertalkers always advise people to check their itineraries constantly. I booked a JFK-SFO RT and chose specific flights because of the 767 configuration (four of us so we can be 2+2 in the 2-3-2 cabin). Without any warning, they switched planes on both flights to 757 so now we're in the single aisle 3-3 cabin, which is not ideal. Luckily, I caught it in time so I could put us back together but we have no choice but sit with random passengers (on my last long flight, the guy next to me in C+ weighed over 300 lbs--talk about torture!).