/**
 * Implements hook_field_update_forbid().
 */
function {{ machine_name }}_field_update_forbid($field, $prior_field, $has_data) {
  // A 'list' field stores integer keys mapped to display values. If
  // the new field will have fewer values, and any data exists for the
  // abandoned keys, the field will have no way to display them. So,
  // forbid such an update.
  if ($has_data && count($field['settings']['allowed_values']) < count($prior_field['settings']['allowed_values'])) {
    // Identify the keys that will be lost.
    $lost_keys = array_diff(array_keys($field['settings']['allowed_values']), array_keys($prior_field['settings']['allowed_values']));
    // If any data exist for those keys, forbid the update.
    $query = new EntityFieldQuery();
    $found = $query
      ->fieldCondition($prior_field['field_name'], 'value', $lost_keys)
      ->range(0, 1)
      ->execute();
    if ($found) {
      throw new FieldUpdateForbiddenException("Cannot update a list field not to include keys with existing data");
    }
  }
}