For a February flight, we purchased B class tickets for DTW to CDG and were upgraded at the time of purchase. When we boarded in DTW, I asked the gate agent if there were seats available if they would upgrade B class tickets at the gate and he said, "Absolutely not!!! We want to sell those tickets."
I don't know if there were any Bs wanting to be upgraded, but there were a few empty seats and at least 2 flight attendants seated in Business.
Do you think Gate Agents give priority to employees?