Drupal 9/10 Change type of fields with existing data

mymodule.install

<?php

use \Drupal\field\Entity\FieldStorageConfig;
use \Drupal\field\Entity\FieldConfig;

function mymodule_update_9001() {
  $database = \Drupal::database();
  $table = 'node__field_name';
  $entity_type = 'node';
  $field_name = 'field_name';
  $new_field_type = 'datetime_timezone';

  // #1 Load storage configuration.
  $field_storage = FieldStorageConfig::loadByName($entity_type, $field_name);

  // If storage config is not found, there is nothing to do.
  if (is_null($field_storage)) {
    return;
  }

  $rows = [];
  // #2 Fetch the data for the old field.
  if ($database->schema()->tableExists($table)) {
    $rows = $database->select($table, 'n')
      ->fields('n')
      ->execute()
      ->fetchAll();
  }

  $new_fields = [];

  // Use existing field config for new field.
  foreach ($field_storage->getBundles() as $bundle => $label) {
    $field = FieldConfig::loadByName($entity_type, $bundle, $field_name);
    $new_field = $field->toArray();
    $new_field['field_type'] = $new_field_type;
    $new_field['settings'] = [];

    $new_fields[] = $new_field;
  }

  // #3 Take the old storage configuration present it as an array and update it's settings.
  $new_field_storage = $field_storage->toArray();
  $new_field_storage['type'] = $new_field_type;

  // #4 Deleting field storage which will also delete bundles(fields).
  $field_storage->delete();

  // #5 Purge field data now to allow new field and field_storage with same name
  // to be created. You may need to increase batch size.
  field_purge_batch(100);

  // #6 Create new field storage, create a new storage based on configuration above.
  $new_field_storage = FieldStorageConfig::create($new_field_storage);
  $new_field_storage->save();

  // #7 Create new fields.
  foreach ($new_fields as $new_field) {
    $new_field = FieldConfig::create($new_field);
    $new_field->save();
  }

  // #8 Restore existing data in the same table.
  if (!empty($rows)) {
    foreach ($rows as $row) {
      $database->insert($table)
        ->fields((array) $row)
        ->execute();
    }
  }
}