Drupal 8 file field to media entities simple migration. Does not move files in filesystem, just creates new media entities.

MODULENAME.install.php

<?php
/**
 * Create media entities from existing file fields.
 *
 * @link https://chromatichq.com/blog/migrating-drupal-file-fields-media-entities-without-migrate-module
 */
function MODULENAME_update_8001() {
  // Nodes types that will get media migrated.
  $node_types = ['article','event','page','session','sponsor'];
  // Map old file fields => new media fields.
  $map = [
    'field_background_image' => 'field_background_img',
    'field_image' => 'field_img',
    'field_metadata_image' => 'field_metadata_img',
  ];

  // Load all of the nodes.
  $nodes = \Drupal::entityTypeManager()
    ->getStorage('node')
    ->loadByProperties(['type' => $node_types]);
  $log = \Drupal::logger('dcavl_media');

  foreach ($nodes as $node) {
    /*** @var $node \Drupal\node\NodeInterface */
    foreach ($map as $file_field => $media_field) {
      // Verify that both the old and new fields exist on the node and that the file field has a value.
      if ($node->hasField($media_field) && $node->hasField($file_field) && !empty($node->{$file_field}->entity)) {
        // Loop through the values and make a list of unique media entity ids.
        // If there is an existing media entity with the file uri, use it.
        // Otherwise create a new media entity from the file.
        $items = [];
        foreach ($node->{$file_field} as $delta => $item) {
          $existing = MODULENAME_load_media_image_by_file_uri($item->entity->getFileUri());

          if ($existing) {
            $log->notice(sprintf('Media entity found for "%s".', $item->entity->getFileUri()));
            $items[$existing] = $existing;
          }
          else {
            $log->notice(sprintf('No media entity found. Creating media entity for "%s".', $item->entity->getFileUri()));
            $entity = MODULENAME_create_media_image_entity_from_file(
              $item->entity,
              $item->alt
            );
            $items[$entity->id()] = $entity->id();
          }
        }
        // Update the media field with the list of media entity ids.
        $node->{$media_field}->setValue($items);
      }
    }

    $node->save();
    $log->notice(sprintf('Updated images for node "%s".', $node->getTitle()));
  }
}

MODULENAME.module.php

<?php

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

/**
 * Creates a media image entity from a file entity.
 *
 * @param \Drupal\file\FileInterface $file
 *   The existing file object.
 * @param string $alt
 *   The image alt text.
 *
 * @return \Drupal\media\Entity\Media
 *   The media entity.
 */
function MODULENAME_create_media_image_entity_from_file(FileInterface $file, $alt = NULL) {
  $media_entity = Media::create([
    'bundle' => 'image',
    'uid' => '1',
    'name' => $file->alt,
    'status' => \Drupal\node\NodeInterface::PUBLISHED,
    'field_media_image' => [
      'target_id' => $file->id(),
      'alt' => $alt,
    ],
  ]);
  $media_entity->save();
  return $media_entity;
}

/**
 * Fetch a media image entity id by uri.
 *
 * @param string $uri
 *   Drupal file system uri.
 *
 * @return bool|int
 *   The found entity id, or FALSE.
 */
function MODULENAME_load_media_image_by_file_uri($uri) {
  $ids = \Drupal::entityQuery('media')
    ->condition('status', 1)
    ->condition('field_media_image.entity.uri', $uri)
    ->execute();

  if (!empty($ids)) {
    return reset($ids);
  }

  return FALSE;
}