There are intermediate solutions for tickets on the day you land, to maintain a bit of flexibility with the low advance fare. On Italo, if you look far enough in advance, you will see there are four ticket price levels. Look at Milan to Venice for Monday, April 22. Under Economy class you will see four types of fare;
EXtra Magic for €9,90
Low cost for €19,90
Economy for €23,90
Flex for £43,90
The same array shows for Prima Class, although the lowest fares may be already gone. Use the little “I” symbol next to the fare name to see the terms. You will see the following for Economy fare:
Economy Supplement 20% Deduction 40%
Change your ticket details as often as you want, at an additional cost, up to three minutes before the original >scheduled departure time. Reimbursable, with charges deducted. In addition, by paying an additional fee, with >the EXTRA TEMPO offer, if you miss the train you have 2 hours to find another seat on the next possible train - >in this event, please speak to Italo staff at the station. Limited offer, subject to availability.
In other words, you can change the time of the ticket by paying the supplement of 20%, which for that €23,90 fare is less than €5.
Only the Magic fare is totally non-changeable.