function custom_bbpress_forum_permalink( $permalink, $post_id ) {
    if ( 'forum' == get_post_type( $post_id ) ) {
        return home_url( '/forums/' . get_post_field( 'post_name', $post_id ) );
    }
    return $permalink;
}
add_filter( 'post_type_link', 'custom_bbpress_forum_permalink', 10, 2 );