Wisconsin Statutes & Annotations
Chapter 971 - Criminal procedure — proceedings before and at trial.
971.29 - Amending the charge.

971.29 Amending the charge.
(1) A complaint or information may be amended at any time prior to arraignment without leave of the court.
(2) At the trial, the court may allow amendment of the complaint, indictment or information to conform to the proof where such amendment is not prejudicial to the defendant. After verdict the pleading shall be deemed amended to conform to the proof if no objection to the relevance of the evidence was timely raised upon the trial.
(3) Upon allowing an amendment to the complaint or indictment or information, the court may direct other amendments thereby rendered necessary and may proceed with or postpone the trial.
When there is evidence that a jury could believe proved guilt, the trial court cannot sua sponte set aside the verdict, amend the information, and find defendant guilty on a lesser charge. State v. Helnik, 47 Wis. 2d 720, 177 N.W.2d 881 (1970).
Since theft is an included crime of robbery, the amendment of the information from robbery to theft did not materially prejudice the defendant. All of the elements of theft are included in the elements of robbery. Of necessity, then, the defendant had notice and opportunity to prepare a defense to the elements of theft as well as to the additional elements that comprise the crime of robbery. Moore v. State, 55 Wis. 2d 1, 197 N.W.2d 820 (1972).
Sub. (2), in regard to amendments after verdict, applies only to technical variances in the complaint, not material to the merits of the action. It may not be used to substitute a new charge. State v. Duda, 60 Wis. 2d 431, 210 N.W.2d 763 (1973).
The refusal of a proposed amendment of an information has no effect on the original information. An amendment to charge a violation of a substantive section as well as a separate penalty section is not prejudicial to a defendant. Wagner v. State, 60 Wis. 2d 722, 211 N.W.2d 449 (1973).
Sub. (1) does not prohibit amendment of the information with leave of the court after arraignment, but before trial, provided that the defendant's rights are not prejudiced. Whitaker v. State, 83 Wis. 2d 368, 265 N.W.2d 575 (1978).
Notice of the nature and cause of the accusations is a key factor in determining whether an amendment at trial has prejudiced a defendant. The inquiry is whether the new charge is so related to the transaction and facts adduced at the preliminary hearing that a defendant cannot be surprised by the new charge since the preparation for the new charge would be no different than the preparation for the old charge. State v. Neudorff, 170 Wis. 2d 608, 489 N.W.2d 689 (Ct. App. 1992).
Failure of the state to obtain court permission to file a post-arraignment amended information did not deprive the court of subject matter jurisdiction. State v. Webster, 196 Wis. 2d 308, 538 N.W.2d 810 (Ct. App. 1995), 93-3217.
That the court's jurisdiction is invoked by the commencement of a case and that the legislature has granted prosecutors sole discretion to amend a charge only prior to arraignment means that the prosecutor's unchecked discretion stops at the point of arraignment. State v. Conger, 2010 WI 56, 325 Wis. 2d 664, 797 N.W.2d 341, 08-0755.
The trial court cannot after trial amend a charge of sexual intercourse with a child to one of contributing to the delinquency of a minor since the offenses require proof of different facts and the defendant is entitled to notice of the charge against him. LaFond v. Quatsoe, 325 F. Supp. 1010 (1971).

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