This was going to be a question but I’'ve found the answer…
There appears to be no way to adjust the connection details (username in particular) if “automatically log in” has been selected. If you’‘re logged in automatically as Joe, there’'s no way to switch to Fred.
After searching in vain for the settings file in ~/Libraries, I finally found it in ~/Spark/settings.xml.
I’‘d raise a feature request but the issue tracker isn’'t allowing new issues from the public.