Not sure about swapping cushions, but releasing the lower seat cover (see YouTube videos about replacing seat cover for directions on releasing seat cover) and adding foam stuffing to worn out and crushed sections was fairly straightforward.
There is information about the best type of foam in other threads, but it was just fairly thin foam sheeting, Used a wood rod to insert the pieces of foam, then pulled the rod back out. Then refasten the seat cover.
Good luck.