This is an unsolved terminology problem across clients. Some friction new nostr users experience is using e.g. their twitter knowledge to what nostr is.

Npub is not a username, as e.g.twitter users are used to. On twitter you can change your username. You cannot change your npub, save for creating a new one, and abandoning your prior npub.

Currently on damus, npub is separate from username. There is also display name.

A new user recently commented about this confusing experience. One thought I passed back to the new user was what if we indicated what is a unique / immutable field, and what is not.