As a General Member, I got a free upgrade to an Economy Plus seat on a KOA-SFO flight that was full. I originally had an aisle regular economy seat, but found that my seat assignment was changed to an aisle Economy Plus seat upon checkin (I was not allowed to select a different Economy Plus seat even though there were some available, though).

Probably means that if you want to buy up to Economy Plus, there are likely to be some available.