Using $wpdb to update current post

Nevermind, I was complicating this whole process because I wasn’t thinking about just using get_post_meta().

add_action('draft_to_publish','gcpl_draft_to_published');
 function gcpl_draft_to_published($post){
      $exp_processed = get_post_meta($post->ID, '_expiration-date-processed', true);
    // check if the custom field has a value
        if($exp_processed != '') {
          delete_post_meta($post->ID, '_expiration-date-processed');
        }
 }