How do you reset sku's?

If you have generated sku’s for a folder and you wind up deleting a few items what are you supposed to do? Is there a way to reset and regenerate the sku’s so the folder is now numbered correctly?

Isn’t he point of SKU to be unique and fixed? If you regenerate your SKU, so some listings now have SKUs previously assigned to other listings, wouldn’t that make it hard to determine what listings belongs to an SKU?