I was never going to try to have the GA try to change anything. This is how I would do it:
4 Pax total:
Pax 1 and 2 have the confirmed F seats and would sit in F on flight 1. Pax 3 and 4 would use pax 1 and 2 F seats on flight 2 while pax 1 and 2 use pax 3 and 4's coach seat on that flight 2.
And yes, there would be a change of gender. If I was going to tell anyone, I would just let the f/a know while getting on board.