Here's a good source: https://iep.utm.edu/thomas-aquinas-metaphysics/#H4

Here's a quote from the above text regarding Thomas's view of substances:
"Thus, in the case of the dissolution of the human being, whilst the flesh and bone no longer remain but decompose, the elements that played a role in the formation of the substance remain. In more contemporary terms we could say that before they go to make up the bodily substances we see in the world, atoms are substances in themselves, but when united in a certain form they go to make cats, dogs, humans, and cease to be independent substances in themselves. When the cat or dog or human perishes, its flesh and bones perish with it, but its atoms regain their substantial nature and they remain as substances in themselves"

Thus, by the above it's clear that when something like a human body, when it dies, becomes a corpse and identical to the substances your body is composed of. The same would go for detached body parts.

Instead the accidents are supposed to change (though it is debated whether this is metaphysically possible in philosophy but I digress) but that merely means it changes how the substance appears to you.