The books from the previous account stay on your kindle. People have done this to download books to their child's kindle and then deregister it so the child can't buy books or see naughty books the parent might have bought. People have also sold their kindles full of books by loading the books then deregistering it. So this can also be used to trade books. I posted a related message here:
http://www.kindleboards.com/index.php/topic,16193.0.htmlunless someone can tell me what's wrong with my proposal...
With regards to it using up a license, Amazon says to get the license back, all you have to do is deregister the kindle.