I was wondering how much retroactive power we would have.

Harry would know, or be able to look up, any number of chemicals that would react very poorly to the open air. It would be exceedingly foolish to carry one on his person, transfigured, such that he would simply have to negate the tranfiguration to have a distraction or attack, but would such a thing be in our power to suggest into existence, if we thought of a sufficiently non-foolish way for Harry to carry this?