When Sony dumped its BroadBand eBook (BBeB) format, it transparently replaced books in that format with ePub for everyone who bought books in that other format. I think Amazon will do the same as books are moved to the new format. As Amazon keeps saying, your books exist on your bookshelf and can be re-downloaded any time. No big deal to switch the formats. For those with older Kindles, they could warn them the new format won't work. Since every Kindle has to be registered with Amazon, they know who has which generation of device.