What constitutes a constructive amendment to an indictment?

A constructive amendment to an indictment occurs "when the terms of the indictment are in effect altered by the presentation of evidence and jury instructions which so modify essential elements of the offense charged that there is a substantial likelihood that the defendant may have been convicted of an offense other than that charged in the indictment." United States v. Morgenstern, 933 F.2d 1108, 1115 (2d Cir.1991) (quoting United States v. Mollica, 849 F.2d 723, 729 (2d Cir.1988), cert. denied, --- U.S. ----, 112 S.Ct. 1188, 117 L.Ed.2d 430 (1992). Constructive amendment of an indictment is a per se violation of the grand jury provision of the Fifth Amendment when such an amendment affects an essential element of the offense. United States v. Roshko, 969 F.2d 1, 5 (2d Cir.1992). Such a violation requires reversal of the conviction, without a showing of prejudice. Id. at 6.