"I could pay my local literature professor to rewrite the end of The Da Vinci Code to better suit my tastes, tear out the pages from my copy of the book and replace them with the improved version, and nobody, nobody would give a flying flip that I had done so."
Yes but you would not have the right to then SELL the book as "The DaVinci Code", for it no longer is that book by that author.
Correct. That's the same conclusion others on this thread helped me reach. It's all about who owns the copy when the editing takes place. That makes sense to me.