Share 'Updating a Kindle book - what happens?'

I currently have short story in kindle format for sale on Amazon. I want to add another short story to the same kindle book. My question is this: what will happen to the existing kindle books that people have bought? I don't want to cheat the original buyers out of a second story. Will their copy of the kindle be updated and how can I be sure or ensure that it is? Hope you can help. :O) Chris