f I recall, it works like UA's. If you choose a seat at booking, and want to move to the Preferred seats, there is a charge. If you risk it and do not choose a seat, and the non-preferred seats are full, you will be placed in a preferred seat at no extra charge.

Also, please note that there are many preferred seats in single aisle planes, but very, very few on International wide-bodies, so I would recommend against the wait and hope strategy for TATL/TPAC flights in Y.