Thank you for your request.
If you notice a delay on your train in Italy, contact the train staff directly. If you have missed your train because of the delay, you need to go to the ticket office at Milano to sort things out on the spot. As a general rule, you cannot take another train without staff confirmation.
A 15-minute changeover time is a little short, if you can leave a little earlier to have more time in Milano, that would be best.
I wish you a nice day.