Drupal 8: Convert text field to paragraph

custom_util.post_update.php

<?php

use Drupal\Core\Utility\UpdateException;
use Drupal\paragraphs\Entity\Paragraph;

/**
 * Convert news,blog,event body field to paragraphs.
 */
function custom_util_post_update_convert_newsblogevent_body(&$sandbox) {

  $bundles = [
    'news',
    'blog',
    'event',
  ];

  foreach ($bundles as $bundle) {
    if ($nodes = \Drupal::entityTypeManager()->getStorage('node')->loadByProperties(['type' => $bundle])) {
      $body_field = 'field_' . $bundle . '_body';
      $content_field = 'field_' . $bundle . '_content';
      foreach ($nodes as $node) {
        // Make sure we actually have both fields.
        if ($node->hasField($body_field) && $node->hasField($content_field)) {
          // Don't try to migrate the body if the content field already has paragraphs in it.
          if (!$node->get($body_field)->isEmpty() && $node->get($content_field)->isEmpty()) {
            $paragraph = Paragraph::create(['type' => 'formatted_text']);
            $paragraph->set('field_formatted_text_body', $node->get($body_field)->first()->getValue()); 
            $paragraph->isNew();
            $paragraph->save();	

            $content = [
              [
                'target_id' => $paragraph->id(),
                'target_revision_id' => $paragraph->getRevisionId(),
              ]
            ];
            $node->set($content_field, $content);
            $node->save();
          }
        }
        else {
          throw new UpdateException('Either body field or content field is missing for ' . $bundle);
        }
      }
    }
  }

}