field_migration.php

<?php

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

/**
 * Change field_number from Number (integer) to Text (formatted)
 */
function field_migration_medium_update_9001() {

  $entityType = 'node';
  $oldFieldName = 'field_number';
  $table = $entityType . '__' . $oldFieldName;
  $revisionTable = $entityType . '_revision__' . $oldFieldName;
  $newFieldsArray = [];

  $database = \Drupal::database();

  $rows = NULL;
  $revisionRows = NULL;
  if ($database->schema()->tableExists($table)) {
    $rows = $database->select($table, 'n')->fields('n')->execute()
      ->fetchAll();
    $revisionRows = $database->select($revisionTable, 'n')->fields('n')->execute()
      ->fetchAll();
  }
  
  // Get field storage config.
  $fieldStorage = FieldStorageConfig::loadByName($entityType, $oldFieldName);

  // Check if field storage config exist.
  if (is_null($fieldStorage)) {
    return t('Field Storage does not exist');
  }

  $newFieldStorage = $fieldStorage->toArray();
  $newFieldStorage['type'] = 'text';
  $newFieldStorage['settings'] = array(
    'max_length' => 255
  );
  
  foreach ($fieldStorage->getBundles() as $bundle => $label) {
    $field = FieldConfig::loadByName($entityType, $bundle, $oldFieldName);
    
    $newField = $field->toArray();
    $newField['field_type'] = 'text';
    $newField['settings'] = array(
      'max_length' => 255
    );
    $newFieldsArray[] = $newField;
  }
  
  $fieldStorage->delete();
  field_purge_batch(40);
  
  // Create new field storage.
  $newFieldStorage = FieldStorageConfig::create($newFieldStorage);
  $newFieldStorage->save();
  
  // Create new fields.
  foreach ($newFieldsArray as $field) {
    $fieldConfig = FieldConfig::create($field);
    $fieldConfig->save();
  }

  // Restore existing data in new fields.
  if (!is_null($rows)) {
    foreach ($rows as $row) {
    $database->insert($table)
             ->fields((array) $row)
             ->execute();
    }
  }

  if (!is_null($revisionRows)) {
    foreach ($revisionRows as $row) {
      $database->insert($revisionTable)
               ->fields((array) $row)
               ->execute();
     }
  }
}