Here's everything you need to know about the developers preview -
All you need to know about the Windows Phone 8.1 'Preview for Developers' | Windows Phone Central
As for updates after WP8.1, we have absolutely no idea. Considering not everyone has WP8.1 yet, probably a little while still.
If you have issues with your phone you can always convert it back using the Nokia Software Recovery Tool prior to sending it it.
FAQ - FAQ - How can I recover/reset/restore my phone software? - Nokia - UK
That's of course if you have any issues before your warranty runs out. Warranty here is only one year.