In Scrivener it’s possible to write in one format and publish in a totally different one without changing the on-screen appearance of a single character. So when you decide you want to change publication formats…say you’ve imported a Word manuscript and published it to Kindle, but now you want to re-format it for CreateSpace…you simply create a new set of formatting instructions using an intuitive compilation wizard, and Scrivener does the line-by-line format conversion for you.
I love Scrivener for much more than ebook formatting, but it's an ace at that. And much more.
(No connection to the wonderful Scrivener folks, just a fan...)