When you republish it all of your readers see it as a new post. I usually say something like “this is from the archives,” but that doesn’t mean much to anyone who subscribed after the original post ran, since the republished post is new to them. By way of example, I might run a post from when I had 300 readers. Today I have more than 3,000. So when I say it’s an old post, it’s actually new to 90% of my audience.