Had the same requirement, moving 550 topics.  The simplest workaround around by:

1. Creating new forum area for destination topics and note forumID shown in wpforo dashboaard for it

2. Use SQL commands in phpadmin to change forumID of topics to move, eg: use regexp topic title names.

3. Sync post forumIDs to match the topic forumID by selecting wpforo Dashboard -> Rebuild threads *after* adding the following code in RED to class-topics.php.  It would make sense for wpforo team to add this to a future release.

function rebuild_forum_threads( $forumid = 0 ){
	    if( !$forumid ){
		    $args = array( 'layout' => 4 );
            $forums = WPF()->forum->get_forums( $args );
            if(!empty($forums)){
                foreach( $forums as $forum ){
                    $args = array( 'forumid' => $forum['forumid'] );
                    $topics = $this->get_topics($args);
                    if( !empty($topics) ){
                        foreach( $topics as $topic ){
                            $this->rebuild_threads($topic['topicid']);
			    // nando4>> when 'thread rebuild' after doing a manual myphpadmin move of topics to a different 
			    // nando4>> we still need to update those topic's posts' forumID 
			   WPF()->db->query( "UPDATE `".WPF()->tables->posts."` SET `forumid` = ". intval($forum['forumid']) ." WHERE `topicid` = ". intval($topic['topicid']) );
                        }
                    }
                }
            }

Thanks for the info. I was going crazy looking for the solution untill I've found this forum.

Thank you!