Change on Italian train ticket reservation???

It depends on the type of ticket you bought. For Trenitalia 'fast" trains, start here, scrolling down to base, economy and super economy fare tiers. Tickets for regionale trains can be purchased just before your journey - so no need to make changes - and their prices are what they are.

https://www.trenitalia.com/en/html/trenitalia/Info_Tariffe_en_LeFrecce.html

For Italo trains, see their fare tiers here:
https://www.italotreno.it/en/the-train/fares

The two classes of discount tickets - Super Economy and Econ - are no change, no refund unless the train company is at fault. However, the Base ticket, can be purchased at any time and is changeable until an hour or so after the train has left the station or before. That is the only ticket class that is changeable. Early or late makes no difference. It is the number of discount tickets available per train. Once sold, no more discount tickets. Generally buying early gives better access to discount tickets but not always. For the Regionale trains it is one ticket, one prices -- no discounts and no reserved seats. Sort of a bus on rails.

Thank you!