Increasing the length of a basefield with data

<?php

/**
 * Change my_basefield max_length to 255.
 */
function my_module_update_10001() {
  $entity_type_id = 'my_entity';
  $field_name = 'my_basefield';
  $field_length = 255;

  /** @var \Drupal\Core\Entity\EntityLastInstalledSchemaRepositoryInterface $schema_repository */
  $schema_repository = \Drupal::service('entity.last_installed_schema.repository');
  /** @var \Drupal\Core\Entity\EntityFieldManager $entity_field_manager */
  $entity_field_manager = \Drupal::service('entity_field.manager');
  /** @var Drupal\Core\Field\BaseFieldDefinition[] $base_field_definitions */
  $base_field_definitions = $entity_field_manager->getBaseFieldDefinitions($entity_type_id);
  $schema_repository->setLastInstalledFieldStorageDefinition($base_field_definitions[$field_name]);
  $field_storage_definitions = $schema_repository->getLastInstalledFieldStorageDefinitions($entity_type_id);

  // Update the serialized schema property.
  $rc = new \ReflectionClass($field_storage_definitions[$field_name]);
  $schema_property = $rc->getProperty('schema');
  $schema_property->setAccessible(TRUE);
  $schema = $field_storage_definitions[$field_name]->getSchema();
  $schema['columns']['value']['length'] = $field_length;
  $schema_property->setValue($field_storage_definitions[$field_name], $schema);

  // Update the field definition in the last installed schema repository.
  $schema_repository->setLastInstalledFieldStorageDefinitions($entity_type_id, $field_storage_definitions);

  // Update the storage schema.
  $key_value = \Drupal::keyValue('entity.storage_schema.sql');
  $key_name = $entity_type_id . '.field_schema_data.' . $field_name;
  $storage_schema = $key_value->get($key_name);
  // Update all tables where the field is present.
  foreach ($storage_schema as &$table_schema) {
    $table_schema['fields'][$field_name]['length'] = $field_length;
  }
  $key_value->set($key_name, $storage_schema);

  // Update the database tables where the field is part of.
  $db = Drupal::database();
  foreach ($storage_schema as $table_name => $table_schema) {
    $db->schema()->changeField($table_name, $field_name, $field_name, $table_schema['fields'][$field_name]);
  }

}