Changing a Trenitalia ticket

I purchased a base ticket and trying to change it and it will only allow me to change the day and not the time. I booked a 7:05 train to Catania from Bari. I want to change it to a train departing 2 days earlier at 18:46. The only option it gives me is the same route and time for the date change and not the time change. Anyone know why I can’t change to the day and time I want?