Updating text field length with existing data in Drupal 11

my_module.post_update.php

<?php

/**
 * @file
 * Post update functions for my_module module.
 */

declare(strict_types=1);

use Drupal\Core\Database\Database;
use Drupal\field\FieldStorageConfigInterface;

/**
 * Increase the maximum length of paragraph subheadline field to 512 characters.
 */
function my_module_post_update_increase_max_subheadline_length(array &$sandbox): void {
  $entity_type = 'paragraph';
  $field_name = 'field_subheadline_text';
  $new_length = 512;
  $value_column = "{$field_name}_value";
  $tables = [
    "{$entity_type}__{$field_name}",
    "{$entity_type}_revision__{$field_name}",
  ];

  // 1. Alter the DB columns directly. Drupal's entity API refuses to change a
  // field's schema once it holds data
  // (SqlContentEntityStorageSchema::onFieldStorageDefinitionUpdate() throws
  // FieldStorageDefinitionUpdateForbiddenException), so we widen the columns
  // ourselves and then sync the tracked definitions below. Widening a varchar
  // never truncates existing values.
  $db_schema = Database::getConnection()->schema();
  $column_spec = ['type' => 'varchar', 'length' => $new_length, 'not null' => FALSE];
  foreach ($tables as $table) {
    if ($db_schema->tableExists($table)) {
      $db_schema->changeField($table, $value_column, $value_column, $column_spec);
    }
  }

  // 2. Sync the installed SQL storage schema so the entity definition update
  // manager stops reporting a mismatch. This is stored in the
  // "entity.storage_schema.sql" key-value collection under the key
  // "{entity_type}.field_schema_data.{field_name}" (see
  // SqlContentEntityStorageSchema::loadFieldSchemaData()).
  $kv = \Drupal::keyValue('entity.storage_schema.sql');
  $schema_key = "{$entity_type}.field_schema_data.{$field_name}";
  $field_schema = $kv->get($schema_key);
  if (is_array($field_schema)) {
    foreach ($field_schema as &$table_schema) {
      if (is_array($table_schema)
        && is_array($table_schema['fields'] ?? NULL)
        && is_array($table_schema['fields'][$value_column] ?? NULL)
        && array_key_exists('length', $table_schema['fields'][$value_column])
      ) {
        $table_schema['fields'][$value_column]['length'] = $new_length;
      }
    }
    $kv->set($schema_key, $field_schema);
  }

  // 3. Update the active field storage config. Use the raw config factory to
  // bypass the entity API (a FieldStorageConfig::save() would re-trigger the
  // forbidden-schema-change check from step 1).
  \Drupal::configFactory()
    ->getEditable("field.storage.{$entity_type}.{$field_name}")
    ->set('settings.max_length', $new_length)
    ->save();

  // 4. Update the last-installed field storage definition so it matches the
  // active config.
  $repository = \Drupal::service('entity.last_installed_schema.repository');
  $definitions = $repository->getLastInstalledFieldStorageDefinitions($entity_type);
  if (isset($definitions[$field_name]) && $definitions[$field_name] instanceof FieldStorageConfigInterface) {
    $definitions[$field_name]->setSetting('max_length', $new_length);
    $repository->setLastInstalledFieldStorageDefinitions($entity_type, $definitions);
  }

  // 5. Clear cached definitions so the new schema takes effect immediately.
  \Drupal::service('entity_type.manager')->clearCachedDefinitions();
  \Drupal::service('entity_field.manager')->clearCachedFieldDefinitions();
}