By going to the very first setting under your settings page, you can change your newsletter's username.
Let's say you change your username from telemachus to telly-weekly. If you had an old email accessible under https://buttondown.com/telemachus/archive/my-chompies, that URL will not continue to work: Buttondown does not automatically redirect old URLs to the new username, for security reasons. However, we do have this functionality hidden behind some internal tooling, so feel free to contact us if you want us to flag it for you.