lydia,
As the others have mentioned, the policy on ticket changes will depend on which category you purchased. Most here don't recommend buying tickets for the fast trains to connect with inbound flights, since the tickets are specific to train, date and departure time and can only be used for the one train listed on the ticket. If you try to use them on another train, you'll be subject to hefty fines which will be collected on the spot!
Here's the deal.....
- Super Economy: non-refundable and non-changeable. If you miss the train listed on the tickets, they're worthless and you'll have to buy more, probably at full fare.
- Economy: these allow one change of date or time prior to the train leaving the station, but a "change fee" is payable.
- Base: These are changeable an unlimited number of times prior to departure without charge, or at the departure station ticket office up to one hour after the train has departed. Booking for a different train is allowed, but a surcharge will apply. Refunds are also allowed, but subject to a deduction.
Even if the tickets could be changed, it likely wouldn't be practical to do so, as you may have to stand in a long queue at the ticket office (in a jet lagged state with luggage) in order to make the changes. Although it costs more, the best solution (IMO) is simply to buy the tickets when you arrive at the FCO airport rail station. You'll also need to buy your tickets for the Leonardo Express which travels from the airport to Roma Termini (where you'll connect with the Freccia train to Salerno). Note that there are two trains servicing FCO so be sure to board the L.E. Also, on that train you MUST validate your ticket prior to boarding, or you'll risk hefty fines which will be collected on the spot!
Finally, you'll only have to deal with Passport control at FCO. That may be fairly quick, but will depend on the number of flights that arrive at the same time and how many staff members they have on duty. "Customs" is simply a walk through the "Nothing To Declare" line and won't take any time at all (unless of course you have something to declare). You may find it helpful to have a look at the excellent Ron In Rome website as there's a good section there on travel from the airport, as well as train travel in Italy in general.