I recently got an op-up while flying LGA-ORD. This has now happened to me three times in the last year on this route. The gate agent just called my name and switched out my boarding pass right before boarding. I think on one occasion, I was flying on an expensive ticket, but generally I have be in Q. I think the only explanation is dumb luck, because I never even spoke with the gate agents before being called up, so it's not like I was buttering them up.