There are many differences between these two file version, but the main difference between these two file extensions is that the XLS is created on the version of Excel previous to 2007 while XLSX is created on the version of Microsoft Excel 2007 and onward.
Another difference between these two files is: the way through which these both store information on it. The .XLS extension is a binary format while that .XLSX is Open XML format.
Now coming to the point, let’s start converting XLSX to XLS,,,,
First you have to open the file that you want to convert, and then click on the File and go to the Save As, and from the list of options select Excel 97-2003 Workbook.
This will save a copy to an old Excel format (.xls format), which you can start sharing.