Whenever you change the email address in this dialog, another confirmation email will be sent. It looks like a way to do email bomb attack…
Discourse identified a user by email. It’s hard to change email address when you are not a user (when you don’t have an identity). Looks like by design IMO.
However, as an logined user, you may change your email address. Or whatever, sign up again with right email address. Taken username is under control of admins. A registered user may talk to staffs to tweak it.