If you already have example.com and sub.example.com configured to be Fully hosted under the same user, you should have both directories visible when you FTP in.
If so, then rename them: example.com to save.example.com
sub.example.com to example.com
save.example.com to sub.example.com
And update your Drupal configuration to use the new directory paths and URL.
The reason I'd stay away from changing the web path is that it may get confusing when you FTP in to your site and the folders don't correspond with the domain they're serving.