My experience is that it happens regularly. A few examples ...
Bumped from row 9 to 28 (last row up against wall) on a 717 from AYQ to ASP some years back, about 20mins before boarding. Something about seating a kid with his dad (why they didn't move the kid back rather than me, highly irksome). Got that sorted at the gate.
Moved from 1A to 1C about 1hr before boarding on flight to Noumea last year. The IFE wasn't working in 1A so I was moved to 1C without knowledge by QF. Gate staff explained why, I asked to be moved back as I wasn't going to use the IFE. On return flight I was called to ask if I wanted to be moved because IFE was still not working in 1A so I said no, I'm OK without IFE on the flight.
Today, moved from 6D to 18D on DHC-8 400 whilst waiting in the lounge. Lounge agent said because of trim as lightly loaded, I accept this which is why every second seat from row 2 to 6 is blocked out on the aircraft on check in, but is all of row 6 now empty? Erm no, I shouldn't have been moved as a WP so I quick call by agent and I was re-instated to row 5.