I am traveling with kids and seniors on LOT later this week. I tried to pay to pre-assign seats using the "My Favourite Seat" link on lot.com, but it only brought me to the usual "Booking Details" page that doesn't allow you to change anything. I sent them a Facebook message and got only this reply:
Hello, selecting seats in economy class is a chargeable service. You can select them here or over the phone. It is always possible to choose a seat free of charge 36 hours before departure, but only from the seats that remain available. //Agnieszka
The "here" in "select them here" was not a link or anything, just that bare text.
I guess I will have to try calling. Pretty disappointing.