As it turns out the reupdate of old ones will automatically start when the 'web imported' attribute is not set, either because the movie has never been imported before or because the attribute was deactivated by user interaction (you). The auto update is triggered even if you set the MagicScipt option for 'Update Movies/People automatically in the background' to OFF. This does seem like a bug to me, but there is a work around : manually set the 'web imported' attribute for all movies and then start doing your desired updates based on selections. During the update the first cycle will deactivate the 'web imported' attribute again but only for the movies you selected for update. The background process does the rest.