Full Idea
Though he retains objects from the 'Categories', in 'Metaphysics' these yield their status as primary substances to their substantial forms. Concrete particulars are now secondary, and that which underlies everything is the substantial form.
Gist of Idea
In 'Metaphysics' substantial forms take over from objects as primary
Source
report of Aristotle (Metaphysics [c.324 BCE], book) by Michael Frede - Title, Unity, Authenticity of the 'Categories' V
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.26
A Reaction
Frede says he moved from realism about substances to nominalism. Presumably substances within objects are real concreta, but forms are abstract, leaving the the object as a purely material thing.