I have an essence - some of my properties are accidental, in that they could be different yet I would still be me; other of my properties are essential, in that were they different it would not be me any more. Surely, the identity of my parents is part of my essence, not my accidents. God cannot make any thing to have a different essence, since that is a contradiction in terms (it is a violation of the law of identity, that A is A). God cannot do this any more than he could create a square circle, or create a rock so heavy that he could not lift it. Insofar as entities have essences, then the power of God does not extend to altering their essences - it extends to choosing whether an entity having that essence exists or not, and then to choosing its accidents. God cannot cause an entity to have a different essence yet remain the same entity, for that is a contradiction in terms.
Simon