If you want to make the last_name empty on convert lead page if it is ‘N/D’ on leads detail view, you can change that on convert lead page in this file custom\modules\Leads\views\view.convertlead.php (create the file and extend the class and override the getRecord method.
require_once('modules\Leads\views\view.convertlead.php');
class CustomLeadsViewConvertLead extends ViewConvertLead
{
protected function getRecord()
{
$this->focus = BeanFactory::newBean('Leads');
if (isset($_REQUEST['record'])) {
$this->focus->retrieve($_REQUEST['record']);
if($this->focus->last_name == 'N/D'){
$this->focus->last_name='';
}
}
}
}