You can change your display name ("David Chartier") but the first part of your actual fediverse address is permanent. It would complicate federation if this changed, it would be like changing an email address, messages would get lost.
The only way you can change the first part is by creating a new account and moving your followers over to that. (This can I think be on the same server? I haven't tried doing an intraserver move though so I'm not 100% sure...)