Now, here's an interesting question :

We, FF, usually chose our seats in advance. Therefore, at check-in, we get the BP with the pre-selected seat. When we get op-uped at the lounge or at the gate, they trash it and give us a new BP with the new seat number.

Now, what happens, if I haven't pre-selected a seat and I'm the last one to check-in ?
At this stage, all seats are assigned and the computer doesn't know if I am eligible for op-up. What seat number appears on my BP ?
Does the check-in agent clearly tell me that my seat will be assigned at the gate ?