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.
this can be considered a problem, but how often does a normal user switches the jabber user?
Normal users are notoriously hard to find.
My scenario:
I’'m originally subscribed as ‘‘jefft’’. It occurs to me this is a dumb name for a customer-facing account
I register ‘‘Jeff’’, and while I’'m at it, auto-login
I notice my new ‘‘Jeff’’ account isn’'t subscribed to all the workgroups. Oh well, back to ‘‘jefft’’
Oops, I can’'t change it
It’‘s trivial to fix this by editing the XML config file, but not trivial to find the config file, since it’'s in a nonstandard place (and I have a lot of junk in my home directory).