Drupal - Hook update to copy values from one field to another - runs after config import

mymodule.install

<?php 

// drush updb ; trigger expection so the update don't pass.
// drush cim ; import your new field.
// drush updb ; the hook update is successfull and values are copied.

/**
 * Populate formatted texts from previous field on paragraph callout.
 */
function mymodule_update_10103(&$sandbox): void {
  $source_field = 'field_text_txtpl';
  $target_field = 'field_content_txtfl';

  $fields = \Drupal::service('entity_field.manager')->getFieldDefinitions('paragraph', 'callout');
  $field_exists = array_filter($fields, function($field_name) use ($target_field) {
    return $field_name == $target_field;
  }, ARRAY_FILTER_USE_KEY);

  if (empty($field_exists)) {
    throw new \Exception('Target field does not exist already.');
  }

  $storage = \Drupal::entityTypeManager()->getStorage('paragraph');
  $paragraphs = $storage->loadByProperties(['type' => 'callout']);

  $langcodes = array_map(function($language) {
    /** @var \Drupal\Core\Language\LanguageInterface */
    return $language->getId();
  }, \Drupal::languageManager()->getLanguages());

  /** @var \Drupal\paragraph\Entity\ParagraphInterface $paragraph */
  foreach ($paragraphs as $paragraph) {
    if ($paragraph->hasField($source_field) && $paragraph->hasField($target_field)) {
      $paragraph->set($target_field, $paragraph->get($source_field)->getValue());
      $paragraph->save();
    }

    foreach ($langcodes as $langcode) {
      if ($paragraph->hasTranslation($langcode)) {
        $trans = $paragraph->getTranslation($langcode);
        if ($trans->hasField($source_field) && $trans->hasField($target_field)) {
          $trans->set($target_field, $trans->get($source_field)->getValue());
          $trans->save();
        }
      }
    }
  }
}