Full Idea
Aristotle's earlier 'Categories' theory of substance, and his later 'Metaphysics' theory, are radically different. The first is realistic, and the second nominalistic.
Gist of Idea
Aristotle moved from realism to nominalism about substances
Source
report of Aristotle (Metaphysics [c.324 BCE], ousia) by Michael Frede - Title, Unity, Authenticity of the 'Categories' V
Book Reference
Frede,Michael: 'Essays in Ancient Philosophy' [University of Minnesota 1987], p.26
A Reaction
Frede claims that 'Categories' is clearly earlier. It is certainly profoundly different from 'Metaphysics'.