Create media entities from existing file fields. Drupal 7 to 8 (9) migration.

mymodule.install

<?php

use Drupal\media\Entity\Media;
use Drupal\file\Entity\File;

/**
 * Create media entities from existing file fields.
 */
function mymodule_update_8801() {
  $converts = [
    [
      'content_type' => ['article', 'game'],
      'media_bundle' => 'image_thumb',
      'new_node_image_field' => 'field_image_thumb',
      'old_node_image_field' => 'field_image'
    ],
    [
      'content_type' => ['humor'],
      'media_bundle' => 'image_humor',
      'new_node_image_field' => 'field_image_humor',
      'old_node_image_field' => 'field_image'
    ],
  ];

  foreach ($converts as $convert) {
    $old_field = $convert['old_node_image_field'];

    // Load all nodes of current CT.
    $nodes = \Drupal::entityTypeManager()
      ->getStorage('node')
      ->loadByProperties([
        'type' => $convert['content_type']]
      );

    $media_ids_remove = [];
    foreach ($nodes as $node) {
      // Verify that the file field has a value.
      if (!empty($node->field_image->entity)) {

        $medias_thumb = \Drupal::entityTypeManager()
          ->getStorage('media')
          ->loadByProperties([
            'bundle' => [$convert['media_bundle']],
            'name' => $node->{$old_field}->entity->label()]
          );

        if (!empty($medias_thumb)) {
          $new_field = $convert['new_node_image_field'];
          $node->{$new_field}->entity = reset($medias_thumb);
          $node->save();

          \Drupal::logger('image_to_media')->notice(
            sprintf('Reuse IMAGE_THUMB for node "%s".', $node->getTitle())
          );
        }
        else {
          $medias = \Drupal::entityTypeManager()
            ->getStorage('media')
            ->loadByProperties([
              'bundle' => ['image'],
              'name' => $node->{$old_field}->entity->label()]
            );

          if (!empty($medias)) {
            $media = reset($medias);
            $file_id = $media->field_media_image->target_id;
            $file = File::load($file_id);
            $new_field = $convert['new_node_image_field'];
            $node->{$new_field}->entity = _mymodule_create_media_image_entity(
              $convert['media_bundle'],
              $file,
              $node->{$old_field}->alt
            );
            $node->save();

            $media_ids_remove[] = $media->id();
            \Drupal::logger('image_to_media')->notice(
              sprintf('Reuse media for node "%s".', $node->getTitle())
            );
          }
          else {
            \Drupal::logger('image_to_media')->notice(
              sprintf('NO IMAGE "%s".', $node->getTitle())
            );
          }
        }
      }
    }

    // Old images will be deleted, to prevent duplicates.
    if (!empty($media_ids_remove)) {
      $storage_handler = \Drupal::entityTypeManager()
        ->getStorage('media');

      $entities = $storage_handler
        ->loadMultiple($media_ids_remove);

      $storage_handler
        ->delete($entities);
    }

  }
}

/**
 * Creates a media image entity from a file entity.
 *
 * @param $media_bundle
 * @param \Drupal\file\Entity\File $file
 *   The existing file object.
 * @param null $alt
 *   The image alt text.
 *
 * @return \Drupal\Core\Entity\EntityBase|\Drupal\Core\Entity\EntityInterface|\Drupal\media_entity\Entity\Media
 *   The media entity.
 * @throws \Drupal\Core\Entity\EntityStorageException
 */
function _mymodule_create_media_image_entity($media_bundle, File $file, $alt = NULL) {
  $media_entity = Media::create([
    'bundle' => $media_bundle,
    'uid' => '1',
    'langcode' => \Drupal::languageManager()->getDefaultLanguage()->getId(),
    'name' => $file->label(),
    'field_media_image' => [
      'target_id' => $file->id(),
      'alt' => $alt,
    ],
  ]);
  $media_entity->save();

  return $media_entity;
}