Derrek Lee had a phenominal year that he will probably never come close to repeating. Being a firstbaseman might hurt his value, but it's never going to be higher anyway, so it makes sense to try and leverage that year into a very good player at a more difficult position to fill, like CF or SS. Even trading him in for a good corner outfielder might not be such a bad idea, although it seems like Lee should be able to handle a corner himself.