Form submission handler for node_assign_owner_action_form().
node_assign_owner_action_validate()
function node_assign_owner_action_submit($form, $form_state) {
// Username can change, so we need to store the ID, not the username.
$uid = db_query('SELECT uid from {users} WHERE name = :name', array(
':name' => $form_state['values']['owner_name'],
))
->fetchField();
return array(
'owner_uid' => $uid,
);
}