Trying to change username after a new site has already been created with another name results in the old name still being used for sftp auth.
Expected result would to be use the new username instead of the old one.
Try fully exiting Opus (via File / Exit Directory Opus) to clear any cached connections.
If it still doesn't work, the connection isn't using the FTP address book at all. e.g. If opening the site via a saved lister layout, it may have saved the connection details rather than a pointer to the address book entry that created them.