Wordpress have many plugin which can help you to export(old blog post) and import(new blog post).
If you don’t want to lose ranking then do a 301 redirection from old blog post to new blog post. The position will drop suddenly when you implement 301 redirection via plugin, but it will come back(90%) after few weeks.
The percentage is 90% because 301 redirection consume some juicyrank/pagerank. So there is no any solution which say your rank will always remain same if you move your content.