They want you to leave your old weapons and get new ones. Thats how these games work. Destiny is still a RPG. Still, whats the point of buying a new expansion if you want to keep your old weapons. Play the old content all over again if you want. Expansions are expansions, meaning that you will grind all over again for better loot. If you could keep your old weapons, there would be no content left in TTK.