Yes, the importer is meant to just "Add on" to what is currently there. Normally you could just wipe out the db and run the install/createtable.php script and everything would be ready for import again. However, since you've done style changes you'll need to do one step prior to that.
Before wiping out the database, you'll want to export your style(s) and then delete them from the control panel. At that point you can wipe the db, run install/createtable.php and then do your import. Then you can reimport your styles.
