The Pros and Cons Of Editing Archived Blog Posts
First, what impact will changing the blog content have? If it's a popular (traffic-driving) post, will making changes force the blog platform to create a new URL? If it does, your readers may not be able to find it and you'll kill a valuable piece of linkbait. Dash says that you shouldn't hesitate when making factual changes, however (like misspelling a source's name or quoting a specific amount of funding), but do alert readers to the change with some kind of "updated on [date] to reflect [change]" description.
Lastly, updating posts can generate duplicate content issues, since if your blog platform creates a new URL, you may have two almost identical posts. There's also some evidence that the engines may penalize an older page that suddenly shows signs of activity--particularly link traffic--at least until there's enough evidence to prove that it's not being used for spam.