Generally they change when the edition is updated or outdated. But, sometimes the prof will give us reading from books that have been out of print for 30 years. That actually only happened in one class when the prof wanted us to read a procedural section on some test. I tend to buy more books than most people I know, but i do talk to the prof a lot. I always ask them, "If you were to buy one book on your list, what would it be?" Then, I will look through the book in the library and decide if I want it. A lot of students will tell you to just read the notes, or read the copy that is in the library, but that won't help you after you graduate. You need to start your library now so when that really wired thing walks into your door, you can pick up a couple text and figure out what it is.
But in general, they don't just change text to change.