I have a staralliance gold card and booked a basic fare on TP. At online check in a day before the flight, I was assigned emergency exit aisle seat for free. Changing it to a back of the bus middle seat would cost me 16 EUR or so And indeed, at T-3, I could change my seat for a window exit row or any other seat for free. Note also that if you travel with a baby, even with a discount ticket, you can freely select your seat although not all seats are suitable for babies.