Some changes are merely accidental or alterational. Others are substantial or existential. It is one thing for Tom to gain or lose weight, quite another for him to come to be or pass away. Alterational changes including gaining weight, shifting position, and becoming depressed. Such changes are changes in a thing that already exists and remains self-same through the change. Call that thing the substratum of the change. It does not change; what changes are its properties. In a slogan: no alterational change without unchange.
But coming-to-exist and ceasing-to-exist also count as changes. Call them existential changes. This prima facie distinction at the Moorean or datanic level between alterational and existential change leaves open three theoretical options: (a) reduce existential change to alterational change; (b) reduce alterational change to existential change; (c) maintain that they are mutually irreducible. (C) is the least theoretical of the three and the closest to the data; let's see if we can uphold it.
Now it seems obvious that existential change cannot be understood in terms of alteration of the very thing that undergoes it: before a thing exists it is simply not available to suffer any alteration, and likewise when it ceases to exist. Coming-to-be is not gain of a property, but gain of a thing together with all its properties; ceasing-to-be is not loss of a property, but loss of a thing together with all its properties. But it also seems obvious that existential change cannot be understood in terms of the alteration of anything distinct from the thing that undergoes it. Thus I don't think that the following tensed definitions of Roderick Chisholm shed any real light on coming-to-be and passing away ("Coming into Being and Passing Away" in On Metaphysics, U. of Minnesota Press, 1989, p. 56):
D1 x comes into being =df There is a property which is such that x has it and there is no property which is such that x had it
D2 x has just passed away =df Something that was such that x exists begins to be such that x does not exist.
Consider the second definition first. If Zeno the cat has just passed away, then the property of being triangular, my house, and me all begin to be such that Zeno does not exist. And conversely. No doubt. But surely the real change which is the ceasing to exist of a cat cannot be understood in terms of mere Cambridge alterations in Platonica or in concreta distinct from the cat. The right-hand side of (D2) cannot figure in a metaphysical explanation of the left-hand side. It is the other way around. The real change in the cat when it ceases to exist is the metaphysical ground of the Cambridge alterational change in the house. Now suppose a cat comes into being. Then of course there is some property that it has, and every property was such that the cat in question did not have it. But again, the real change that occurs when a cat comes into existence cannot be understood in terms of Cambridge alterations of properties.
So Chisholm's definitions, though true, shed no light on the metaphysics of coming-to-be and passing-away. Real existential change cannot be understood in terms of Cambridge changes.
But if Zeno's coming to be cannot be understood in terms of (D1), why can't we say that his coming to be is just the alteration of the gametes whence he sprang? Creation (exnihilation) aside, coming to be is coming to be from something that already exists. So why not say that when a substance comes to exist it comes to exist by the alteration of an already existing substance or substances?
Consider the house of the Wise Pig. It is made entirely of bricks. It came to be from those bricks. Assume that each brick is an Aristotelian primary substance. Did a new Aristotelian substance come into existence when the assiduous pig changed a pile of bricks into a house proof against the depredations of the Big Bad Wolf? Or did nothing new come into existence? It would be reasonable to hold to the latter view and maintain that all that happened was that an alterational change occurred to the bricks. Similarly when the house is disassembles. Nothing passes out of existence. You have what you started with, a loa of bricks.
It is different with cats and people. For example, when a person dies, its body is altered in various ways; but if the person ceases to exist at death, its ceasing to exist is not identical to the person's body being altered in these ways. A rational substance ceases to exist. And the same holds when a person comes into existence either at conception or some time thereafter. This coming into being cannot be identified with the alteration of such already existent material particulars as the mother's uterus and its contents. A rational substance comes to exist. Generation and corruption, to use the not entirely felicitous Aristotelian language, are at least in some cases irreducibly existential changes. Whether or not the coming to be of the Wise Pig's brick house is an addition to being, a person's coming to be is. (On the Boethian definition invoked by scholastics, a person is a primary substance of a rational nature.)
If a person's coming to be is a change, it is an existential change. It is not an alterational change in an existing substance or in existing substances. Nor do persons spring into existence ex nihilo. Persons develop from nonpersons and in such away that the nonpersons cease to exist and the person begins to exist. But if all change requires a substratum of change that remains self-same through the change, a substratum that provides continuity and ensures that the change is a change and not a replacement, what the devil is the substratum in the case of coming-to-be and ceasing-to-be? The Aristotelian-scholastic answer is prime matter. Prime matter, however, though its postulation is well-motivated by a couple or three different lines of argumentation is arguably unintelligible. Prime matter is a wholly indeterminate and wholly formless really existent stuff of which all material substances are composed. It belongs wth G. Bergmann's bare particulars and Kant's Ding an sich in point of unintelligibility or so I would argue.
More on materia prima later.