Italia Rail from florence SMN to rome termini

if you paid for a discounted ticket (Economy or Supereconomy fare) there are restrictions on your ability to make changes. Basically you will have to pay for the full fare, plus a penalty (if you chose the option to be able to make changes with a penalty, otherwise no changes allowed).

Thanks. Our tickets are 'standard'. One specific aspect of my question is whether there is an old-school "window" at the station where i can walk up and discuss this kind of change?

Both Trenitalia and Italo have manned ticket offices at Florence SMN. ItaliaRail is not a train company. It is merely a 3rd party ticket seller.

Steve, you should have been able to see the restrictions on your tickets when you purchased them. Italiarail is also just a 3-party ticket seller and not the rail service you'll be using to Florence; that'll be either Trenitalia or Italo. It's likely Trenitalia in a Standard carriage but it's the price tier than determines the restrictions. Is there anything on your tickets that could tell us more?

But yes, there are manned ticket booths at Firenze SM Novella.

(Edited to clarify "Standard')

"Standard" does not describe the fare you paid.
In Trenitalia parlance "Standard" describes the class of seats, like Economy in airplanes. The classes of seat on a Frecciarossa are (in increasing order of comfort): Standard, Premium, Business, Business Silence (no talking loud on phones), Business Salottino (small conference rooms for business meetings), Executive.
The fares can be: BASE (full refundable fare), ECONOMY, SUPERECONOMY, FRECCIAYOUNG, FRECCIADAYS, etc. Except for the Base fare, all others have restrictions on the ability to get a refund/change.