In the Dashboard of your WP.com blog, under Tools select Export. Create an XML file containing your posts and comments for you to save or import into another WordPress blog.
Download this file to your PC.
In your new blog, in the Dashboard under Tools, elect import and upload the XML file.
WordPress.com offers a very affordable Site Redirect. It’s only $12 and you can find it under store in your WP.com Dashboard.
Good luck!