Well this link says you can get licenses back
http://www.kindleboards.com/index.ph...;topic=18105.0
though they had to both delete the book from the device and sync it to remove the book. Whether this was necessary or just a coincidence? Would deregistering it work just as well even with the book left on the kindle.
There are also lots of threads on the internet saying you can only reset your 6 device licences by calling amazon.
Who knows....