How to set a private file path and move files in one pass

I'm writing an update function to set the file_private_path variable and then move files to the private location. This is what I have so far:

// Define path variables
$private_path = conf_path() . '/files/private';

// Set the private path variable and create the directory
variable_set('file_private_path', $private_path);
drupal_mkdir($private_path);
file_create_htaccess($private_path, TRUE);

// Find certain managed files
$file_query = db_select('file_managed', 'fm') 
  ->fields('fm', array('fid'))
...<snip>...
$file_result = $file_query->execute();

// Move all the files to the private file area
while($record = $file_result->fetchAssoc()) {
  $file = file_load($record['fid']);
  $file = file_move($file, 'private://');
}

But file_move fails because "because the destination private:// is invalid." If I split the process into two scripts, where I first set the variable, and then do the file_move, it works ok. So I guess Drupal needs to bootstrap again after file_private_path is set so that it knows where "private://" is. Is there something I can do to allow this all to happen in one step?

Thanks in advance for any tips.