What constitutes a constructive amendment to an indictment?

"A constructive amendment [of the indictment] occurs" when the evidence presented at trial and "the district court's jury instructions combine to 'modify essential elements of the offense charged to the point that there is a substantial likelihood that the defendant may have been convicted of an offense other than one charged by the grand jury.'" United States v. Vebeliunas, 76 F.3d 1283, 1290 (2d Cir. 1996) (quoting United States v. Clemente, 22 F.3d 477, 482 (2d Cir. 1994)). Such amendments "are per se violations of the fifth amendment that require reversal even without a showing of prejudice to the defendant." Id.