What is the structure of the "Featured Image" field in the DB?

You should probably not run preg_replace on the meta data for a _wp_attachment_metadata field as the data is serialized. I would recommend instead you write a small script to loop through the images, unserialize the data, update the value, serialize and store the modified array. Here’s a sample function which does just that:

function wpse_135525_fix_imagepaths() {
    $images = get_posts(array(
        'post_type' => 'attachment',
        'post_mime_types' => 'image',
        'posts_per_page' => -1,
        // Extra filtering, e.g. by post_date etc.
    ));

    foreach ($images as $image) {
        $attachment_meta = get_post_meta($image->ID, '_wp_attachment_metadata', true);
        if ($attachment_meta) {
            $attachment_meta = unserialize($attachment_meta);

            $filename = $attachment_meta['file'];
            $filename = preg_replace('/(\d)\1/', '$1', $filename); // Replace any two repeating digits (44 => 4, 11 => 1 etc.)

            $attachment_meta['file'] = $filename;

            update_post_meta($image->ID, '_wp_attachment_metadata', serialize($attachment_meta)); 
        }
    }
}

You’ll probably want to limit the post query as to only process attachments created within a defined period of time, and the regex should need some improvement to prevent it from replacing any repeating digits in the first part of filenames etc.

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