My point was that you might want the URL to stay the same, but swap out what it points to - which I am not sure you can do here. That way if people pass around the URL outside your control, it remains up to date. Sure you can update the link, but if someone has already coppied and pasted it, it's too late.
I'm not sure I follow, but I want to... this thread is pinned, so if people link the thread it should stay good for a long time. If they share the hyperlink to the file itself, it will probably go bad when I update the PDF. Like you said, I don't know if I can do much about that. You can still find it by plugging "Magic for the Masses" into a search engine, though.