What is a constructive amendment to a criminal charge?

More important, this court has held that "a constructive amendment occurs when the crime charged [is] substantially changed at trial, so that it [is] impossible to know whether the grand jury would have indicted for the crime actually proved." Pisello, 877 F.2d at 765 (quoting United States v. Van Stoll, 726 F.2d 584, 586 (9th Cir.1984)). We have observed that those cases in which a constructive amendment has been found have "dealt with verdicts resting on proof so different from the proof accepted by the indicting grand jury that substantial rights of the defendant were affected." Id. at 766.