What constitutes a constructive amendment to a criminal charge?

In United States v. Von Stoll, 726 F.2d 584, 586 (9th Cir.1984), we said 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." The court added, however, that a mere variance occurs when the prosecution's evidence proves facts different from those alleged in the indictment. See id. The line essentially is between the situation in which different evidence supports the charged crime and that in which the evidence supports a crime other than that charged. Although the latter, an amendment, requires reversal, the former, a variance, does not warrant reversal unless it affects the substantial rights of the defendant. See id. at 587.