Trenitalia ticket changed to train + replacement bus, but no notification?

Your current ticket is still valid. Just go with the flow. Regional tickets are often open tickets, good until validated. It is virtually impossible for Trenitalia to notify everyone who bought tickets, as many buy them anonymously out of ticket machines.

Thank you for the info.

Question: when I’m getting ready to head back to Palermo and I go to the Agrigento train station, I assume I’ll be able to find the buses fairly easily and that they’ll have signs, correct?

I am sure that would be correct. Remember, you are not the only one. Other people will be doing likewise.

We had a very similar experience in September…we had regionale tickets from Firenze to Lucca showing a direct trip (with stops butno change). The train stopped in Pistoia, and because the four of us were in the last car with just two other people, we didn’t notice that EVERYONE had left the train. I finally peeked out the door and found a Trenitalia employee who basically said, “Are you looking for the bus?” Huh? He checked my ticket and apologized profusely, called the bus and told them to wait for us. Because I had purchased the tickets several months prior to our trip, the changes (adding a new parallel rail line) didn’t show, nor did they show when I checked our tickets the day prior to travel. Our tickets were honored, as I’m sure yours will be, but it might be a good idea to check with a ticket agent at the rail station.

We were on a regional train last Fall from Orvieto to Rome and the arrival station got changed from Termini to Tiburtina. We were never notified even though we purchased our tickets online and they had our contact info, and we had the train app. I only figured it out once we were on the train and reading the display of stops. Apparently there was maintenance at Termini that closed some tracks. Sensing a pattern : ). Grateful that the train wasn't canceled; since we'd been traveling all day we just got a cab to our hotel, instead of walking from nearby Termini as planned.

The strange part is that my existing ticket still only shows Palermo
Centrale to Agrigento Centrale. It does not mention the bus or the
transfer.

Since the ticket is still for a trip from Palermo to Agrigento one would indeed expect to still show that.

Tickets aren't for a train. They are for a route. And when you buy a ticket it is actually expected that you check what is running on the route a day or so before travel, and adjust your plans accordingly. And as Trenitalia has no idea whatsoever who will all be on that train they don't bother with notifications.

Also it was not necessary to buy this ticket in advance. This is mass transit. Tickets are always available, and in this case you can buy up to 5 minutes before departure.

When you say "tickets are always available", do you know if that means they continue to sell tickets even though all seats are reserved/purchased, so there is always standing room? Or are you saying that in general regionale trains are never sold out?

We got shut out of a train in Japan the year before where "tickets are always available, no need to purchase in advance", and nearly missed our connecting train which is known to sell out, so were trying to be more cautious by making sure we had a seat on the Italian train(s).

so were trying to be more cautious by making sure we had a seat on the Italian train(s).

There are no seat reservations on Italian regional trains, so the possibility you have to stand, at least until people get off and seats become available, is a possibility. On the other hand, Italian highspeed trains and IC trains have seat reservations required that come with the ticket purchase.