Hi RK,
Thank you very much for your interest in Syncfusion products.
Essential XlsIO supports to read the XMLSpreadSheet files and save it to Excel 2010 formats. We have prepared the sample at our side to re-save the Excel 2.1 sample file (SpreadShetML - Excel 2003) to Excel 2003(.xls), Excel 2007(.xlsx) and Excel 2010(.xlsx) formats. Please try the attached sample at your side and let us know if these helps you.
Please go through the following online documentatio line for opening various excel formats through XlsIO.
http://help.syncfusion.com/ug_92/Reporting/XlsIO/Windows%20Forms/default.htm?turl=Documents%2Fworkbook.htmPlease go through the following online documentation link for various file support formats in Essential XlsIO.
http://help.syncfusion.com/ug_92/Reporting/XlsIO/Windows%20Forms/default.htm?turl=Documents%2Fintroductiontoessentialxlsio.htmPlease let us know if you require any further clarifications.
SpreadSheetML_fa1ce9cf.zip