Sometimes the outdated posts with good position in search engines can become a delicated problem, what you do with your outdated posts receiving good traffic. The kind of posts you can't modify because the software / script / trick is no longer working or available this days or no longer necessary. I don't have such posts but i asked myself this question before starting a real blog.
I can suggest very good thing here. Create a new post and then redirect all that traffic to your newly created post.