Originally Posted by
rankourabu
They will rebook you into the lowest available class (thats not lower than your original) on the next flight. Simply, if the flight only had Y seats for sale, thats what you get rebooked into and thus treated as a Y customer.
But that by itself will not get you the bonus qualifying miles. NW tracks fare paid/fare flown. If they are a bit harried or sloppy when you are rebooked, they will lose track of your original fare paid and the flight can end up posting as B/B or Y/Y.
W to Y is even more
exciting ....