Missouri Revised Statutes Title XXXV. Civil Procedure and Limitations § 512.300. Statement of account amended, when

In all cases of an application for trial de novo, the bill of items of the account sued on or filed as a counterclaim or setoff, or the statement of the plaintiff's cause of action, or of defendant's counterclaim or setoff, or other ground of defense filed before the associate circuit judge, may be amended upon a trial de novo to supply any deficiency or omission therein, when by such amendment substantial justice will be promoted; but no new item or cause of action not embraced or intended to be included in the original account or statement shall be added by such amendment. Such amendment shall be allowed upon such terms as to costs as the court may deem just and proper.