Hi,
The main product can't have the same product code than the variant, it's working like that in HikaShop.
But I think that you are talking about the product name, which is small when on the main product, and detailed when selecting a variant.
I have done a little modification in the view "product / show_default" I have commented a part like that:
// if (hikashop_getCID('product_id')!=$this->element->product_id && isset ($this->element->main->product_name))
// echo $this->element->main->product_name;
// else
echo $this->element->product_name;
With this modification, the product name is always detailed, even if on the main product.