I would say yes, it does happen but not common. I have been cancelled on maybe two flights over the past 6 years that had very low occupancy that suddenly developed "mechanical problems"...but I have also been on many flight that were only 20-30 people.
If they have another fully booked flight that is having problems or delays...then I could see it happening for them to get ahead, but I would guess that they do not cancel flights simply for low capacity.
I have been on the other end of that situation with Alaska airlines where they cancelled another flight and gave us their plane because ours was having mechanical problems. It was not intended but the way the gate agent made the calls was blatantly obvious they we just stole their plane and they got bumped!
And this is not just a SW thing...any and every airline would do the same.