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 set
if ($temp[0] != 0) {
//unset all properties that aren't a #tree to perserve hierarchy
foreach (array_keys(element_properties($form['taxonomy'])) as $property) {
if ($property != '#tree') {
unset($form['taxonomy'][$property]);
} 
}
$form['taxonomy']['#type'] = 'value';
$form['taxonomy']['#value'] = $temp;
}
}