I have 2 tickets booked for November in S class. Does anyone know if it is possible to pay the fare difference (one way) from the S class fare to the instant upgrade P class fare after ticketing? Is there a penalty? Has anyone done this before and been successful?

I know I will have to call AA to do this, but wanted to see if it was even possible before calling. I am always afraid to call on an existing reservation because of the chance of the rep causing a problem with the tickets already purchased.

Thanks all.