Hi Nooghoso86 @RomanoS
Buses have also global prices, as all other long distance trains in Italy. The ticket is valid only for a specific bus. However, you have no assigned seat.
I made a test and I found an important information. The 16:00 bus on 25 Apr. is already sold out, this is why I really recommend to exchange the reservation:

I don’t think that train reservations are automatically transferred to the bus, the departure times are not the same. It is true that the railways must transport you, but you have no guarantee that you will arrive in Zermatt on the same day. In worst case you would have to spend a night in Milan or travel via the Gotthard.
You should also consider that a bus ticket from Milan to Domodossola costs only 16 € on trenitalia.com. I would buy such tickets to be sure to be able to travel and arrive on time in Zermatt.
It is impossible to say if a refund of your original ticket may be possible or if it is better to use it from Domodossola to Zermatt, it depends if it is a saver or a regular ticket. A problem is that if you bought the ticket in the SBB app, the Trenitalia ticket office at Rome has no access to it. However, they could probably give you new reservations free of charge, if the buses are not sold out…