I am currently reading "Real Essentialism" written by David Oderberg. He discusses the issue of substance individuation. The principle of individuation is matter. Now if we have Plato and Socrates, then they share the same form (of course understood in abstraction) but they differ in matter. Now he presents the problem discussed by Fine where Plato is eating Socrates at breakfast and slowly the matter of Socrates replaces the matter of Plato. After this process Plato consists of the same matter as Socrates, therefore Plato and Socrates are identical.
Oderberg solves this problem by adding a time index to matter. Then Plato consists of the same form and matter as Socrates, but at different times, so they are not identical after all.
But why assume that they consist of the same matter in the first place? Socrates is unified substance. He is matter formed by his substantial form. Now when Plato is eating Socrates, then (assuming he is alive) he gradually loses his matter. But then detached matter is substantially changed (it is no longer formed by Socrates' form and it has, for a short period of time, its own separate existence). So first some "chunk of matter" is part of Socrates (formed by his substantial form), then it is formed by some other form, and next it is formed by Plato's form.
But substantial changes seem to exclude the possibility of talking about the same matter, because we have different subjects of predication.