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It will not have any real effect other than changing your on page SEO. If the dates are changed, and content is updated with a good portion of information, the post may get ranked higher just because of freshness. Google loves fresh content and fresher content usually rank better but fall a little below later on. What you may try is making a new post with a link referring to the old post. See if that works for you and if not you can always update the content on the original post...which is what I would do. Keeping content updated with changes is a good way for your users/readers, not just for SEO Good content and updated content can have others point back to your site, i.e. "hey check this page out, they update this information quite often!".

Thanks guys for the quick responses.

Thanks for the response Peter.