Check if value has changed on save_post

In the end I checked against and updated a new meta value on each save.

  $screen = get_current_screen();
  if ( $screen->base == 'post' && $screen->post_type == 'sessions') {
  if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) return;

    if ( isset( $_POST['session_status_tax'] ) ) {
      $status = $_POST['session_status_tax'];
    } else {
      $status="";
    }

    $prev_term = get_post_meta( $post_id, 'prev_term', 'true' );

    if ( $status === 'status-approved' &&  $status !== $prev_term ) {
      write_post_log($post_id, 'Approved' );
      send_email( $post_id, 'Approved' );
    }

    update_post_meta( $post_id, 'prev_term', $status );

 }
}