Update post tags

gistfile1.txt

add_action( 'init', 'update_post_tags' );

function update_post_tags() {
    $q = new WP_Query( array( 'post_type' => 'post', 'posts_per_page' => -1 ) );

    if ( $q->have_posts() ):
        while ( $q->have_posts() ): 
            $q->the_post();
            $post_id = get_the_ID();
            $args = array( 'fields' => 'names' );
            $categories = wp_get_post_categories( $post_id, $args );
            $cat_names = implode( ', ', $categories );
            wp_set_post_tags( $post_id, $cat_names, true );
        endwhile;
    endif;
}