Never happened to me until today. 
I saw my seat bounce back and forth from an upgrade to a downgrade twice before my flight. 
Not a big deal since it's only a 45 minute flight, but it was interesting to see the upset flyers at the counter, wondering what happened to their first class upgrade.

Gate agent came over the PA claiming they had to switch planes, so a lot of people had seat changes. I think that's false since the seat configuration stayed the same. 
I'm guessing someone bought last minute first class seats this morning and they had to move us freeloaders further back.