/**
 * Implements hook_field_storage_update_field().
 */
function {{ machine_name }}_field_storage_update_field($field, $prior_field, $has_data) {
  if (!$has_data) {
    // There is no data. Re-create the tables completely.
    $prior_schema = _field_sql_storage_schema($prior_field);
    foreach ($prior_schema as $name => $table) {
      db_drop_table($name, $table);
    }
    $schema = _field_sql_storage_schema($field);
    foreach ($schema as $name => $table) {
      db_create_table($name, $table);
    }
  }
  else {
    // There is data. See field_sql_storage_field_storage_update_field() for
    // an example of what to do to modify the schema in place, preserving the
    // old data as much as possible.
  }
  drupal_get_schema(NULL, TRUE);
}