Creating what does not exist and changing natures to something new

If the words of Elijah had power even to bring down fire from heaven, will not the words of Christ have power to change the natures of the elements? You have read that in the creation of the whole world he spoke and they came to be; he commanded and they were created. If Christ could by speaking create out of nothing what did not yet exist, can we say that his words are unable to change existing things into something they previously were not? It is no lesser feat to create new natures for things than to change their existing natures.

St. Ambrose, from treatise on the Mysteries

This is such a basic bit of logic and yet one that never occurred to me. Now I just have to hope it pops into my mind if I am in discussion about the Eucharist.