If you have communicated with somebody using just a username what happens when you change your username? Do they get notified of the change so they can still reach you or do you need to tell them the new username?
Discussion
changing your username does not delete your current chats or contacts.
"a username is not the profile name that’s displayed in chats, it’s not a permanent handle, and not visible to the people you are chatting with in Signal. A username is simply a way to initiate contact on Signal without sharing your phone number."