Method to cleanup SilverStripe $Content from old installations on-the-fly using HTMLPurifyer

CleanupContent.php

<?php

/**
     * cleans up $Content using HTMLPurifyer
     *
     * @return \SilverStripe\ORM\FieldType\DBHTMLText
     */
    public function getContent()
    {
        $content = $this->dbObject('Content');

        $config = HTMLPurifier_Config::createDefault();

        $config->set('CSS.AllowedProperties', []);
        $config->set('AutoFormat.RemoveEmpty', true);
        $config->set('AutoFormat.RemoveEmpty.RemoveNbsp', true);
        $config->set('AutoFormat.RemoveSpansWithoutAttributes', true);

        $purifier = new HTMLPurifier($config);
        $clean_html = $purifier->purify($content);

        return \SilverStripe\ORM\FieldType\DBHTMLText::create()->setValue($clean_html);
    }