You need to specify the file format explicitely. This works for me:
private void buttonExcel97_Click(object sender, EventArgs e)
{
string safeExcelFilePath = @"c:\temp\cp.excel";
string excel97File = Path.Combine(safeExcelFilePath, "Excel97.xls");
string excelOpenXMLFile = Path.Combine(safeExcelFilePath, "ExcelOpenXML.xlsx");
Microsoft.Office.Interop.Excel.Application _excel = new _Excel.Application();
Workbook workbook1 = _excel.Workbooks.Open(excel97File);
workbook1.SaveAs(excelOpenXMLFile, XlFileFormat.xlOpenXMLWorkbook);
}
For detail information have a look here:
XlFileFormat enumeration (Excel) | Microsoft Docs[
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