Originally Posted by
MatthewLAX
I would have just tried to board with the Passbook BP, then when it beeps at the gate you play dumb, but firmly insist on your original flight in your original seat.
+1
There are circumstances when playing dumb is the best possible course of action - not knowing the intricacies actually makes it harder for a GA to explain how a pax with a BP for a flight present at boarding time is being denied for any reason other than IDB.
I'd add that, having had my seat changed by GAs a couple times in the past, I always stop and get a paper BP from an airport machine - nothing beats a real paper trail.