Very complete article in wphub.com with tips to export and import , from the default WordPress import/export functionality, to a long list of plugins for that matter, and also included script to import to MySQL database from Excel or CSV.
I haven’t really tested all of it, but I think it’s worth to check this out when you need to do this kind of work.
You have installed the plugin WordPress SEO by Yoast, went to SEO -> XML Sitemaps, clicked the button where it says “You can find your XML Sitemap here: XML Sitemap” and… oh no! Error 404 Page not found !!
What to do? easy peasy, Setting -> Permalinks and re save your permanent links by clicking on Save Changes.