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if ($form_id == 'forum_node_form') {//we use 2 because that is the taxonomy id for "forums"$temp = $form['taxonomy'][2]['#default_value'];//if a forum has been setif ($temp[0] != 0) {//unset all properties that aren't a #tree to perserve hierarchyforeach (array_keys(element_properties($form['taxonomy'])) as $property) {if ($property != '#tree') {unset($form['taxonomy'][$property]);} }$form['taxonomy']['#type'] = 'value';$form['taxonomy']['#value'] = $temp;}}