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.
Basically means:
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.