The *section* itself would be a derived work. But would the *changes*? If that's the case, copyright law is more stupid than I thought. If I take a copyrighted work and add my own ending to it, does that mean that later I cannot use that ending on an independent work?
No !
Your own a copyright on your addition. You just can't use the derived work of the copyrighted text (the copyrighted text + your addition). Logically, the author of the original text can neither use the derived work. In fact, nobody can use the derived work without the authorization of the other author.
You can use your addition by itself if you want. It's not a derived work in itself, only the combination of the 2 texts is.
Jean-Baptiste Soufron Intellectual Property and Contracts