Sorry, sounds like you're SOL.

For future travelers who will read this:

It is absolutely unnecessary and often costly to purchase this type of ticket through RailEurope. RailEurope operates zero trains. They are a ticket reseller (usually for a profit). You can buy tickets directly from the operator.

You can purchase tickets from Trenitalia (one operator) for 43 euros per adult. Those tickets have FREE AND UNLIMITED changes before departure. You can buy these, in English, on their website (http://www.trenitalia.com) or in person at the station.

You can purchase tickets from Italo (another operator) for 45 euros per adult for FREE AND UNLIMITED changes before departure, or 37 euros per adult for changes that require a supplemental charge. You can buy these, in English, on their website (www.italtreno.it)

Depending on which stations are convenient to you.

Folks, RailEurope is rarely, if ever, a better deal than working with the travel operator directly, and rarely, if ever, more flexible. American (and other) guide books do travelers a real disservice by recommending them.