As a former (decades ago) agent, first of all, I wouldn't release people's seats before we even boarded. That's going to lead to dupe seats and ultimately more work. For sake of argument, let's say I did do this. I would "un upgrade" the pax for the one who paid for the seat. It's the right thing to do. Hopefully this could be handled before boarding. After boarding it's a lot less "comfortable" for everyone.
I have to wonder if looking through the optics of the Dao/UA incident hasn't changed airline policy in this situation.