change text field length when data is present

mymodule.insall.php

/*
 * Utility to change the max length of a text field
 * @see http://nathan.rambeck.org/blog/42-modify-drupal-7-text-field-maximum-length
 *
 * @param string the field name to update
 * @param integer the new field length
 */
function mymodul_change_text_field_max_length($field_name, $new_length) {
  $field_table = 'field_data_' . $field_name;
  $field_revision_table = 'field_revision_' . $field_name;
  $field_column = $field_name . '_value';

  // Alter value field length in fields table
  db_query("ALTER TABLE `{$field_table}` CHANGE `{$field_column}` `{$field_column}` VARCHAR( {$new_length} )");
  // Alter value field length in fields revision table
  db_query("ALTER TABLE `{$field_revision_table}` CHANGE `{$field_column}` `{$field_column}` VARCHAR( {$new_length} )");

  // Update field config with new max length
  $result = db_query("SELECT CAST(`data` AS CHAR(10000) CHARACTER SET utf8) FROM `field_config` WHERE field_name = '{$field_name}'");
  $config = $result->fetchField();
  $config_array = unserialize($config);
  $config_array['settings']['max_length'] = $new_length;
  $config = serialize($config_array);
  db_update('field_config')
    ->fields(array('data' => $config))
    ->condition('field_name', $field_name)
    ->execute();
}