Can I change the SKU on an existing Amazon listing?

Unfortunately, Amazon does not allow you to change existing SKUs - unless you remove your existing listing and create a new one.

The SKU is used as a key identifier in your feeds, similar to the Item ID on eBay: When you change a price on eBay, you tell eBay the new price for a particular Item ID - but when you change a price on Amazon, you tell Amazon "This is the new price for the SKU ..."

So in order to change the SKU on an already listed item, you can either visit seller central to delete the SKU and then delete the item from WP-Lister's database - or you use the "Remove from Amazon" bulk action and wait until your product removal feed has been processed.

See Deleting Amazon listings for more details.