Actually, "half-eaten lasagna turned into soup" sounds more like you made a casserole or a stew than a soup. Soup usually has a broth or thin liquid base, and leftover lasagna layers might get mushy but they don't really break down into broth. That sounds more like you just reheated it too long and it fell apart into a saucy, layered mess. Did you add any extra liquid or just let it sit in the pan?