Buddypress Topic Mover by dwenaus allows BuddyPress group mods and admins to move a single forum topic to another group.
Sometimes your users may inadvertently place a forum topic under the incorrect group in BuddyPress. This plugin helps moderators and administrators move those topics to the correct groups. The list of groups to move to is loaded dynamically via ajax to cut down on page load times for large BuddyPress installs. The plugin also plays nicely with the Anncounce Group plugin, so users can only move a topic to an announcement group if they are a moderator or admin of that announce group.
Download and more info: http://wordpress.org/extend/plugins/buddypress-topic-mover/