I just went through this. I have flown very little over the last few years due to job changes and a general boycott of air travel due to the TSA and dropped from plat to dirt. I am flying tomorrow and could not get a seat assignment as all the non-preferred seats were filled. OLCI just opened and it showed many bulkhead, exit and premium aisle/window seats open.

Checking my reservation said I could pay $29 for any of these seats at check in.

When I started OLCI, the computer assigned me 11D (on a 738) and did not try to charge me. It also would not let me select any other seat. I'm just happy I'm not stuck in a middle seat.