Vijayan Hi,
In Italy, tickets are always train/time bound and seat reservations are mandatory on long distance trains. It you have a point-to-point ticket without any discount, you can exchange the reservation free of charge, online if you buy or load the ticket in the Trenitalia app. You can also make the exchange at a ticket office.
It were also possible to inform the train staff if you are not in the right train, but you have to pay a fee of 10 €, As indicated earlier, only for tickets without discounts.
Some discounted tickets are exchangeable, but you have to pay an upgrade to the BASE price. Look at the conditions before buying such tickets.
I would recommend to buy the tickets in advance because you have now replacement buses between Domodossola and Milan with a limited number of seats.
Kind Regards
Danhiel