You have both a ticket and a reservation. Tickets are for a route, reservations are for a particular train. If a train gets cancelled that does not invalidate your ticket, so you still have a Zurich - Veniec ticket, and the railways still have to transport you.
The normal procedure in case of disruptions in Italy is to go to the service desk at the station usually at the head of the platform.
In this case the railways have been a step ahead of you and have already informed you what train to take. That is an improvement. So just do that.