It sounds like you are using a OW award issued by AA ... and those do not allow any routing changes, only date changes. You must fly all sectors in the original sequence and on the airlines as ticketed. You can rebook using the 800 number. They can endorse the date change on the PNR so that the paper tickets do not need to be reissued (though this might vary by airline, and presumably you are flying IB which tends to be sticky on paperwork). So by the rules you have to fly to FCO and resume your ticket for a later date (when award inventory is available). I don't know what flexibility AA might have to make an exception to the rules for a voluntary reroute (this is no problem for involuntary reroutes, but you don't have that situation).