From Trenitalia:
Base Fare:
Before departure you can change your Base ticket free of charge and unlimited times; after departure, up to 1 hour, free of charge once at train ticket office at the departure station only.
(goes on to explain online change options)
Economy Fare:
With the Economy ticket, you may only change the date or time of departure once, up to the departure time of your train, by selecting a journey on the same category of train or service type and then paying any price difference there may be when compared to the originally purchased full Base price.
Then there's Super Economy, not changeable. There's a new tiRimborso option that makes more of these refundable when you pay extra at the time of purchase:
With this option, you can request a refund up to 2 days prior to the departure of your booked train, subject to a 10% deduction to the original amount paid.