If you're willing to do a bunch of work, you may be able to recreate something close to the original KFX. AFAIK you won't get kerning though. That seems to be tied to the renderer used for KFX files.
The other part is that there isn't a single definitive KFX file for a title. If you have an Oasis and I have a Paperwhite and we both download the same book, I believe we get different KFX files. There's no reason to doubt that there will be future Kindle devices that are even better with respect to typography and so you will need to re-download your library for that device.
Today, KFX can sometimes be De-DRM'd but it's gotten to the point where it's a cat and mouse game. It works for a while then Amazon changes something and breaks it. I don't doubt that essentially uncrackable DRM is achievable.