When you edit the post, you can delete the old macro from the post, and add the new one. And that's what people will see. You can change anything you want, including images. So you don't have to worry about people downloading an older version.
The forum actually keeps track of all edits, including files and images, although nobody but you and the admins can see the previous edits. So you can revert to the previous edit if you need to. I needed to do this once when I realized I made a mistake in my new macro. Fortunately I realized it right after I made the edit, so I just rolled it back to the previous edit until I could fix it.