Move WordPress pre-existing attachments to S3 using WP Offload S3 Lite.

Example.php

<?php

/**
* You can include this snippet anywhere.
* Just remove the reference once you're done.  
* It checks to make sure files exist and are readable.
* 
* If WP Offload S3 Lite is not installed, this won't do anything other than waste resources.
*
* Love the full plugin?  Buy it!
**/

add_action('aws_init',
    function ($aws) {
        global $wpdb;
        $ids = $wpdb->get_col('SELECT ID FROM '.$wpdb->posts.' WHERE post_type="attachment" AND ID NOT IN (SELECT post_id FROM '.$wpdb->postmeta.' WHERE meta_key="amazonS3_info") ORDER BY RAND() LIMIT 5000');
        $i = 0;
        foreach ($ids as $id) {
            $data = wp_get_attachment_metadata($id);
            if (!is_array($data)) {
                $data = json_decode($data, true);
            }
            $file = WP_CONTENT_DIR . '/uploads/' . $data['file'];
            if (
                !file_exists($file)
                ||
                !is_file($file)
                ||
                !is_readable($file)
                ||
                is_dir($file)
            ) {
                continue;
            }
            wp_update_attachment_metadata($id, $data);
            $i++;
        }
    }
    , 99, 1);