If you have PYOBP then you are unlikely to be told of the problem until you get to the departing gate, unless you visit a lounge beforehand. In which case you can ask for an alternative option. Otherwise you'll get a bleep at the gate and then be given a new BP with a new seat, and at that point the agent will probably be loathed to re-seat you unless you kick up a fuss. But you can't do anything online.

The App boarding pass can change dynamically, so you can see it happen.