My experience with this is... it can be all over the place. When the price is good, go for it.
I ended up paying more by waiting to decide.
Booked flights for OGG last December. Flights in July. At time of booking, PC was $109.
By the time decision to upgrade was made, outbound was 139 and inbound was 119. It has gone to 129 each way and now up to 149 each way. PC is almost filled with a few middle seats open.
Not sure if those are a party of two who chose aisle and window , hoping middle will be kept open. They May end up with stranger in middle seat. 🙄
interesting, elite or full fare seats not taken, at all, on both flights.