[@mwcreative Participant 8 years, 5 months ago]

Just in case anyone needs to alter the forum of topics manually with PHP:

1. Find the Forum ID you’re moving topics to.
2. Create a function to search for your required topics and set the ‘post_parent’ as the forum ID. (sample below)
3. Set the topic’s post meta fields ‘bbp_topic_forum’, ‘_bbp_topic_forum’ and ‘_bbp_forum_id’ as the Forum ID.
4. Repair the forums (tools->forums)

After these steps you should see your topics in the correct forum!

Place in functions.php file of your child theme and modify to your needs, then call the function:
** Disclaimer! I’m not responsible for breaking your site! backup your install before using this code to change your topics! **

function mwc_change_forum_topic_ties() {
        
        // 18 was my target forum ID.  get yours.
	$forumid = '18';
	
	$args = array(
		'post_type' => 'topic',
		'post_status' => 'publish',
		'posts_per_page' => -1,
	);

        /* uncomment this to filter your topics by something

        $oldForumID = '10'; //change this to target a specific forum
        $args['meta_query'] = array(
                'meta_key' => 'bbp_topic_forum',
                'meta_value' => $oldForumID,
                'compare' => '='
        );

        */
	
	$query = new WP_Query( $args );
	
	if($query->have_posts()):
		while($query->have_posts()):
			$query->the_post();
			$theid = get_the_ID();
			
			$theargs = array(
				'ID' => $theid,
				'post_parent' => $forumid
			);
			
			wp_update_post($theargs);
			
			update_post_meta($theid, 'bbp_topic_forum', $forumid);
			update_post_meta($theid, '_bbp_topic_forum', $forumid);
			update_post_meta($theid, '_bbp_forum_id', $forumid);
			update_post_meta($theid, '_bbp_topic_id', $theid);
			
		endwhile;	
	endif;

}