The bugzilla web pages don't seem to have a way to change an existing login to use a different username without just creating a new username. In particular, I had a bugzilla account as meissner, and when Red Hat, Inc. bought cygnus, I would like to change my account to meissner....
The admin of Bugzilla can do this, along with anybody who has been given rights to edit members by the admin (editmembers.cgi). All you have to do is find that person (probably dkl).
https://bugzilla.redhat.com/bugzilla/userprefs.cgi currently allows changing username.