anything is possible but site policy is to limit the point at which such a change can be executed - if you have passed a threshold on number of posts made it won't happen ever
reasoning is that the more you have posted the more you have created a presence for yourself on fp and to suddenly change your username would be to remove continuity/contiguity of the spot/thread paradigm; more simply it would cause confusion, especially if members are searching for posts/threads you made under the old username
@throne: contact me via pm we do not do this in public