Converting multiple .xls format files to .xlsx format

I am new to Linux. I know that there are ways to convert between file formats in Linux through the terminal. But is there any way I achieve the following? I have 3 folders, inside those folders, there are 15 more folders. Each of those folders has 12 files in .xls format, and some other .R files and pdf files. Is there a way to create an exact duplicate of these 3 folders, inside which all the .xls are converted to .xlsx format? If the following seems too unsubstantial, what is the way to just convert all the .xls files in one folder into .xlsx file format? Then I can replicate the aforesaid. note: I don't have LibreOffice, I use WPS. If the following requires LibreOffice I can install it if necessary.