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If the content is simply duplicated then moving it over with a 301 redirect is the best way to go. If, however, you've got separate, unique content, you may need to export those posts (or copy + paste) from the old Wordpress install into the new one before the shift or you could lose old content from the prior install. I'd probably recommend contracting a Wordpress developer you trust or who has some experience (many contractors can be found on sites like oDesk, Freelancer, etc.) and asking them to help with the file move if the content isn't duplicated. Best of luck!