Understand, If there's was LOWER berth available at the time of booking, then you would have got it automatically. As the passenger mentioned by you got confirmed berth, other than the LOWER berth. It's highly likely that the LOWER berth was not available at that time. Now, in my opinion, don't cancel your booking for a LOWER berth. If you really want to have a LOWER berth, then first check the availability status and then proceed with your re-booking plan availing the berth preference as a LOWER berth.