3
AFAIK, if you change the name of a module's directory, you also have to go into that module's xoops_version.php file and alter the $modversion['dirname'] line to reflect the change. They have to be set exactly the same.
i.e.
.\modules\newdirname
[xoops_version.php]
$modversion['dirname'] = 'newdirname';
As Dave said though, if all you want to change is the way it's displayed on the menu, you can just alter the name through the admin modules screen.