I now found out what is planned: They are doing some work on a tunnel in the section on the HSL between Orvieto and Chiusi. Both the old line and the new fast line actually pass though Orvieto, and just north of it there is a link that allows trains to switch from the new line to the old line, then in Chiusi they can switch back.
The old line carries the IC and regional services, the new line the FR services. The deviation probably adds something like 20 minutes to Roma - Firenze trains, and all trains now must use the same tracks between Orvieto and Chiusi. And since the FR trains come from Napoli or further, and continue beyond Firenze they basically need a complete new timetable for the whole route. And Trenitalia is not know for being quick with that…
My suggestion is to just wait till mid June, and then look again.