This can sort of be done by administrators by messing with the database at the heart of the phpBB forum (if they have access, that is).
However, it is not really satisfactory as the database structure is not properly normalised and after a change of username the old name will be left behind at lot of places in the database, and, of course, inside posts written by other users.
Far better to pretend to be a new person and just reregister under a new username (but don't tell Lukand 'cos he'll be worried by the increase in the number of registered but inactive users
)