Difference between old and new state

I am developing a SOAP server that should send state changes to clients. The server communicate with other systems to update its internal state.

I am searching for a solution (pattern o whatever) that let me track the state changes after an update. I watched to memento pattern but it is not what I am looking for, because I don't need the previous state but the change. I can't simply propagate the change to clients because its a pull architecture (clients poll for changes).

Use case

Suppose the server state is a list of object with 3 elements and the update modifies it adding a new element. The client must receive only the newly added element and not a list with 4 elements.

Any solution?