@Rosy: if you change how authors are displayed in calibre to LN, FN then the folders will also change to that order.
The
Quality Check plugin can do the change for you. However, it does it by guessing what is a last name and can easily get it wrong.
You can do the job manually with total control using the Manage Authors dialog (right-click on Authors in the tag browser, choose Manage Authors). What I would do is use the Manage Authors "Copy author sort to author" button to change all the authors, then go down the list fixing any that it got wrong.
Once you make this change you will likely need to fix any new author you add to your library.