If you are using PowerStore v2, then you should be able to change the SKU for each options. Basically on your product update page, you should see the product options listed on the top of the page. By default, the default option is shown, but if you want to change the sku for a different option, just select the option from the list and update the sku and then click the update button. This will update the SKU for that specific product options.