One data point. My wife booked small city-ORD-LGA with return EWR-small city all in one reservation (Q fare). When booking, price to purchase First Class was $250 more, although the only flight with First Class was ORD-LGA. Just after booking, the pop-up box offered the upgrade for ORD-LGA for $119. That segment became Z fare with double PQMs and 1.5 PQSs. As expected, no extra PQDs.