Originally Posted by wallcraft
Content that is designed around the new features won't work on an older Kindle, but much of it will likely be best on a color screen anyway. This is also true of the Kindle PDFs, which are typically optimized for big screens.
Amazon already includes the original ePub in their AZW ebooks, and it looks like this might be replaced by KF8. In other words, an AZW ebook will include MOBI and KF8. From the FAQ:
Given that KF8 is trying to be ePublike, it will work fine on eink screens.