As also the wolves said when Perrin watched Rand's epiphany on Dragonmount, if Dark-Rand hand't become Zen-Rand, there wouldn't be any Last Battle. It would be the end of everything. But Aviendha's visions made clear that Rand even won it. Thus Rand's change to the Light again was obviously already incorparated in Aviendha's future. Still, Aviendha has to change something else, so that the conflict between Seanchan and Aiel will be resolved in another way.