The object of deny is any full reality. The word change is being used as a noun, and is the object of the preposition to, which indicates whose claim is being denied. (Note that this in not an instance of the infinitive to change.) So this:
Classical ontology denies any full reality to change.
Classical ontology denies that change is fully real.
The antecedent of the relative pronoun which is the noun change. So the second half of the quoted sentence means:
Change is conceived as only accidental and not essential.