I just copied my songs from old computer to new MacBook Pro, and iTunes gave me a warning that I can only do that one more time. It said I had already put my music on three computers before this one (I only have two other computers). What happens when I throw out those computers and get new ones? After carefully building a big music library for many years, am I to throw out everything after I upgrade computers one more time? There must be a way to de-register the iTunes from my old computers. How would I do this? Thanks.