Remove “%” from strings in serialized arrays in wp_postmeta

General idea would be to loop through all attachments to retrieve, modify and write back their meta.

Something like this (test thoroughly before using on anything important):

$posts = get_posts(array(
    'post_type' => 'attachment',
    'numberposts' => -1,
));

foreach( $posts as $post ) {

    // retrieve data, unserialized automatically
    $meta = get_post_meta($post->ID,'_wp_attachment_metadata', true);

    // do stuff with $meta array

    // write it back
    update_post_meta($post->ID, '_wp_attachment_metadata', $meta);
}