Assuming that meta_value is **always** dd/mm/yyyy and you are looking for yyyymmdd (which isn't clear), try this:

    UPDATE wp_postmeta 
    SET meta_value = CONCAT(SUBSTRING(meta_value FROM 7), 
         SUBSTRING(meta_value, 4, 2), 
         SUBSTRING(meta_value, 1, 2);

Run that as a SELECT first to make sure it's what you're really looking for.