In the past couple of weeks I've had communications with a few other authors about how you go about changing the story codes after you post the story. If you're uploading a new chapter or a repost of a chapter you can do the changes as part of the file upload process. However, it's easier to just use your Author/ Editor page to make the changes whenever you want.
I've created a short instruction and it's available to anyone at my Dropbox account via the link below. I hope this helps someone.