My recommendation is to always install a major subsystem such as Wordpress into its own directory under the site home. That way it's cleanly separated from site general files (error documents, robots.txt, .htaccess, etc.) and from other subsystems you may install in the future. You either install a "landing page" at the site root, with links to the subsystems, or (so long as Wordpress is the only subsystem installed) use an .htaccess URL rewrite to invisibly shift a visitor from the root into the Wordpress subsystem.

As you will be shifting an existing WP installation from one directory to another, it would be primarily a matter of copying over all the files in the WP tree to another directory (preserving the tree structure). You might even be able to just drop the add-on (did you mean subdomain?) and leave its directory (say, /blog) and all the files in place. Either way, there's bound to be at least one WP configuration file containing the blog's URL and path to its root, that needs to be updated. I'm not familiar with the details of WP, but for SMF there is a "repair_settings.php" utility to guide you through this process. Ask on the WP support site if there is such a tool for WP, or if not, what things need to be manually updated to move an installation, or scan all files for the URL and paths used in the installation. There will probably be no reason to make any changes to the database itself, but there might be URL- or path-dependent information stored somewhere in the database that would need to be updated too.