Well one way to do it is to delete the subdomain via cpanel then create an addon domain using the specified folder.

The other way to do it is to create a new cPanel account and upload the files to that.

But I think this is all overly complicated when you could have just hosted the new domain that they wanted and built the site on the new domain. Its not like robots or humans are going to stumble across the new site, and if they do, so what?