Grab the user number of them both, then go to member management and click the merge user link (I'm not sure if it's on the member management page directly or if it's on the edit user screen).
I think the old attachments want their old directory path, you could try setting a redirect in your .htaccess file; post both full url's and i'll make you a quick line for it.
To re import you'll export your styles, then drop ALL mysql tables, then run "createtables.php" from your install directory then run your importer. All of the critical settings to run an install are set in your config file, but a lot of customization settings are stored in the db.
