Aspose.Cells unique feature that provide flexibility in version conversions without affecting work. SaveFormat is enumeration that can convert document in the extensions given below in table.

Member Name Value Description
CSV 1 Represents a CSV file.
Xlsx 6 Represents an xlsx file.
Xlsm 7 Represents an xlsm file which enable macros.
Xltx 8 Represents an xltx file.
Xltm 9 Represents an xltm file which enable macros.
TabDelimited 11 Represents a tab delimited text file.
Html 12 Represents a html file.
MHtml 17 Represents a mhtml file.
ODS 14 Represents a ods file.
Excel97To2003 5 Represents an Excel97-2003 xls file.
SpreadsheetML 15 Represents an Excel 2003 xml file.
Xlsb 16 Represents an xlsb file.
Auto 0 If saving the file to the disk,the file format format accords to the extesion of the file name.
If saving the file to the stream, the file format is xlsx.
Unknown 255 Represents unrecognized format, cannot be saved.
Pdf 13 Represents a Pdf file.
XPS 20 Represents an XPS file.
TIFF 21 Represents an TIFF file.
SVG 22 Represents an SVG file.
Dif 30 Data Interchange Format.
Below is code snippet that shows conversion from xls to xlsx you can do it vice versa as well
 Workbook workbook = new Workbook("Sample.xls");

workbook.Save("Converted.xlsx", SaveFormat.Xlsx);

Download Sample Code