When a user is created with a username, it is not possible to change the username anymore. However, a new user can be created for you with the new username and the old user can be deleted if needed.
When you delete a user with the user role 'restricted client user' or 'authoriser', this does not have any effect on the documents. The documents are safely stored in the archive and will not be deleted.
A restricted client user can see all documents that that they deliver themselves to Basecone. They cannot see documents of another user.
An authoriser can see documents that they have approved in the archive. They cannot see any other documents in Basecone.
So when the old user is deleted and a new user is created, the new user will not be able to see the 'old' documents in the archive anymore. Because it is a new user, they will only be allowed to see the new archive.
Helpful article:
How can I delete a user in Basecone?