As i said before they adjust the capacity according to the recorded demand.
For example JFK, JNB and LAX still have 9P or 8P.
It has nothing to do with unions.
Once on board you will be able to change seats and i m sure that if you ask at the lounge they will do everything they can to satisfy your demand.
Technically it is impossible to have two different cabin layouts 8P and 5P. So the system blocks out 3 seats. Furthermore the airline has to take this into account in order to establish the crew roster
About one month before departure if there is a P pressure on the flight , they cabin layout will swap to 8P/9P.
The same will happen on the day of departure.
As a P customer you can be sure that even if the 5P flight is full you will get a confirmed seat which means that the flight will swap to 8/9P.
Sales are not restricted.