fixed when replacing existing rows. previously only appending worked.

CopyRowOpenXML.cs

using System;


using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;
using DocumentFormat.OpenXml;
using DocumentFormat.OpenXml.Packaging;
using DocumentFormat.OpenXml.Spreadsheet;

namespace CopyRowOpenXML
{
    class Program
    {
        static void Main(string[] args)
        {
            // backup the file incase we corrupt the file
            string fromFile = "C:\\Users\\Joseph\\Desktop\\sample.xlsx";
            string toFile = "C:\\Users\\Joseph\\Desktop\\sampleResult.xlsx";
            if(System.IO.File.Exists(toFile)){
                System.IO.File.Delete(toFile);
            }
            System.IO.File.Copy(fromFile, toFile);
            // modify the backup
            using (SpreadsheetDocument spreadsheetDocument = SpreadsheetDocument.Open(toFile, true))
            {
                CopyRowsAndReplace(spreadsheetDocument, "Sheet1", 3, 5, 7);
                spreadsheetDocument.Save();
            }
            Console.WriteLine("Done");
        }

        /// <summary>
        /// Copies the rows in the document using the rows starting from 'srcRowFrom' until 'srcRowTo' (inclusive) and places them starting at 'destRowFrom'.
        /// If there is data already in those rows, it will be overwritten.
        /// </summary>
        /// <param name="document">The document to take the rows from</param>
        /// <param name="sheetName">The name of the sheet in the 'document' to take the rows from</param>
        /// <param name="srcRowFrom">The index of the row to start copying from</param>
        /// <param name="srcRowTo">The index of the final row to copy</param>
        /// <param name="destRowFrom">Where to start placing the copied rows</param>
        public static void CopyRowsAndReplace(SpreadsheetDocument document, string sheetName,
               int srcRowFrom, int srcRowTo, int destRowFrom)
        {
            WorkbookPart workbookPart = document.WorkbookPart;
            if (srcRowTo < srcRowFrom || destRowFrom < srcRowFrom) return;
            int destRowFromBase = destRowFrom;
            WorksheetPart worksheetPart = GetWorksheetPartByName(document, sheetName);
            SheetData sheetData = worksheetPart.Worksheet.GetFirstChild<SheetData>();

            // 1. Delete the existing rows if any are present.
            int destRowTo = destRowFrom + (srcRowTo - srcRowFrom);
            IList<Row> rowsToDelete = sheetData.Descendants<Row>()
                .Where(r => r.RowIndex >= destRowFrom && r.RowIndex <= destRowTo)
                .ToList<Row>();
            foreach(Row row in rowsToDelete)
            {
                row.RemoveAllChildren();
            }

            // 2. Copy the rows
            IList<Row> rowsToCopy = sheetData.Descendants<Row>()
                .Where(r => r.RowIndex >= srcRowFrom && r.RowIndex <= srcRowTo)
                .ToList<Row>();

            int currentCopiedRow = destRowFrom;
            foreach (Row row in rowsToCopy)
            {
                Row copiedRow = new Row() { RowIndex = new UInt32Value(Convert.ToUInt32(currentCopiedRow)) };
                foreach(Cell cell in row.Elements<Cell>())
                {
                    Cell copiedCell = new Cell() { 
                        CellReference = new StringValue(GetColumnLetter(cell.CellReference.Value) + currentCopiedRow.ToString())
                    };
                    copiedCell.CellValue = new CellValue(cell.CellValue.Text);
                    copiedRow.Append(copiedCell);
                }
                sheetData.Append(copiedRow);
                currentCopiedRow++;
            }
        }

        private static WorksheetPart
           GetWorksheetPartByName(SpreadsheetDocument document,
           string sheetName)
        {
            IEnumerable<Sheet> sheets =
                document.WorkbookPart.Workbook.GetFirstChild<Sheets>().
                Elements<Sheet>().Where(s => s.Name == sheetName);

            if (sheets.Count() == 0)
            {
                return null;
            }

            string relationshipId = sheets.First().Id.Value;
            WorksheetPart worksheetPart = (WorksheetPart)
                 document.WorkbookPart.GetPartById(relationshipId);
            return worksheetPart;

        }

        private static string GetColumnLetter(string cellReference)
        {
            Regex regex = new Regex("[A-Za-z]+");
            Match match = regex.Match(cellReference);
            return match.Value;
        }

    }
}