The easiest way is to delay your return for one day. Just tell an agent that this might be the best solution for your situation caused by the schedule change. As mentioned earlier, United can force open save business awards on its own metal for you.
Alternatively, if you have found award availability of A* partner flights from FCO to another European city served by United (e.g., FRA, LHR, CDG, ZRH), United can also force open save business awards on its own metal from this particular city to a US hub and then your destination. Basically, you have to do some homework before you call United again.
Good luck.