Not quite. If the substance changes, the substantial form must change. It's the accidents, not the forms*, that remain unchanged.

Woops, you're right, that was a slip. I meant "form" there in the non-philosophical sense: physical apperance. 

Bear in mind too that the Eucharist is officially a mystery

That seems reasonable to me, but then I suppose I wonder why Catholic dogma insists pretty to know precisely what it means. I once asked a well-educated protestant why he disbelieved in transubstantiation, and he said that it was because the doctrine is too specific; he did not know what it meant for the bread to be the body, but for that exact reason he made no specific claims about knowing what it meant. Catholics on the other hand not only make very detailed claims about what happens in the Eucharist, but they insist that belief in those claims are central to the faith (dogma). So I guess I'm unconvinced by claims of mystery.

Belief in the Real Presence is a gift of faith, not one at which we can arrive through unaided natural reason.

I worry that this undermines our ability to know the world we live in. If bread can be be God in substance, can other objects be completely different substancesunbknownst to us? It oddly opens the door to the claims of the skeptics.

because in the conversation described in John 6 Jesus commanded us to do so as a vehicle of sanctifying grace

Does Catholic theology contain an explanation for this command? That is, how it goes about providing sanctification? 

Hm. That's a lot to take in, but I think your bolded section is the part to skip to for discussion.

at the ontic level, the level of being, the relation of causality is the opposite: the consecrated host is substantially the body of Christ precisely because the soul of Christ is its substantial form.

This seems philosophically uninteresting. Are you saying that the material that makes up the bread has the substance of Christ because it has the substance of Christ? That's sounds circular, unless I misunderstand what you're asserting. 

In general, your arguments seems to obliterate any meaningful connection between form and matter. You write that "a living organism is the body of Peter because the soul of Peter is its substantial form, i.e. because it is "informed" (in the hylomorphic sense) by the soul of Peter, i.e. substantially joined to it." But by the same token the soul is also dependent on the body: Peter's soul could not inform just any hunk of matter (a door, say), but rather only that specific body. Think of a rubber ball: its form includes roundness and bounciness, but these attributes are specific to that grouping of matter and could not inform other groupings such as bricks or trees. 

As far as I can see, our apparent ability to make such predications doesn't undermine Aquinas's primary point, which is that we don't have any direct knowledge of God's essence, what God is in and of Himself.

What if we rephrase the predication to "God, in and of Himself, has an essence such that he can take on a human nature (though the means of incarnation remain a mystery to us)"? For that matter, why not "God's essence, in and of itself, is such that finite creatures can't make non-analogous predications about it"? 

We could always ask him. But it doesn't matter; some analogies are closer than others, and I deliberately chose one that was rather a stretch just to make the point.

Asking him doesn't help at all, because the point is that no one knows, not even him, whether the analogy is helpful for conveying what redness really is. He can't compare his idea of redness to the actual thing, and we can't compare actual redness to his idea. So there's an unbridgable gap. And  this point holds even if we talk about a better analogy: If one were to call a sloppy person a "pig" the anology is very apt, but if I just don't know what a literal pig is then it doesn't matter how much of a stretch or not the analogy is, it simply can't convey useful information to me.

"As far as I know, Aquinas didn't contend that we could understand Christ's human nature only analogically. And the Incarnation itself is still a mystery, so even if we understand univocally that Jesus did this or that, we still don't understand univocally how it is that God did this or that."

But if we predicate things about Christ's human nature, aren't we predicating something about God? We can predicate about God that he is such a being that his person can have a human nature and we mean that he can have it in a sense univical to how we have it (and it must be univical to how we have it, or else he has not assumed humanity in the way necessary for our redemption).

Consider (to adapt a fairly well-known example) a blind man who is told that "red is like the sound of a trumpet." He doesn't have a univocal understanding of red, and yet the analogical understanding seems to be effective enough for his purposes.

How could we or he know that it is "effective for his purposes"? We have no way of knowing whether he has gained any knowledge about the color in itself that actually corresponds to reality, nor does he. The analogy only works for the sighted--and then only weakly--because we have prior knowledge of redness and the sounds of horns. It seems the blind man might just as easily gain a misconception about the nature of redness when given such an analogy; and couldn't we as well gain grace misconceptions about God if we really are like a blind man being given analogies of color?

I suppose then my follow up question would be what is it about the angels' acts of existence that properly indivduates them? They're all identical in form, and it is, according to Feser, their individuals act of existence that makes them distinct. If they're all identical in form, it seems like they would all pursue the same set of actions, so I'm not sure how their existence, even in this more active sense you describe, would be different. 

Thanks again for the help!