XLS and XSLX are basicaly two different file formats, so changing the extension won’t change the format.
Which library are you using to generate the excel files? Maybe there is an option to configure the output format?
Alternatively, you can create an additional java action which converts XLS to XLSX using apache poi.
https://stackoverflow.com/questions/14533544/xls-convert-to-xlsx-using-java-and-poi-apache