Full Idea
In 'Categories' primary substances are individual concrete objects, such as a particular horse, whereas in 'Metaphysics' such things are combinations of matter and substantial form, with the latter being the primary substances.
Gist of Idea
Earlier Aristotle had objects as primary substances, but later he switched to substantial form
Source
report of Aristotle (Categories [c.331 BCE]) by E.J. Lowe - The Possibility of Metaphysics 9.1
Book Reference
Lowe,E.J.: 'The Possibility of Metaphysics' [OUP 2001], p.190
A Reaction
Lowe claims there is no real difference. Aristotle came to think that matter was not part of primary substance, so the shift seems to be that substance was concrete, but then he decided it was abstract. Physicists will prefer 'Metaphysics'.