Originally Posted by
golfingboy
In that case, it will only be J inventory, they will not sell C/D/Z/P if is actually booked full. In those cases, the fare class will tell you that you won't get an upgrade.
This is actually not true. I regularly see flights (like UA938 on 11/23) that are F4 A4 J3 C3 D3 Z3 P3, yet business is already at capacity and there are exactly 4 F seats unassigned. UA is willing to sell *ANY* premium fare (by way of overselling the J cabin) to fill 3 out of the 4 seats. Only one seat is "held back" for a real F (or A) purchase.