How to migrate "textfield" to "telephone" (D7 to D10)

A more efficient approach would be to proceed with the migration from Drupal 7 to Drupal 10 as it is. Once the migration is complete, you can create a new "telephone" field in Drupal 10 and then transfer the data from the old field to the new one.

For example, if your content type is "article," and your existing field is `field_old` while the new telephone field is `field_new`, you can use a script to copy all the data.

$batch_size = 50;
$nids = \Drupal::entityQuery('node')
  ->condition('type', 'article')
  ->execute();
$nids_batches = array_chunk($nids, $batch_size);

foreach ($nids_batches as $batch) {
  $article_nodes = \Drupal\node\Entity\Node::loadMultiple($batch);

  foreach ($article_nodes as $article_node) {
    if (!$article_node->get('field_old')->isEmpty()) {
      $article_node->set('field_new', $article_node->get('field_old')->getValue());

      $article_node->save();
    }
  }
}

@marc.bau, I hope you resolved this. Changes are made on on 11.x (our main development branch) first, and are then back ported as needed according to the Core change policies.

There are several support options listed on our support page (Community > Support at the top of Drupal.org) and there is Drupal Slack. You may get better replies in one of those places. I suggest using the #migration channel in Drupal Slack.