See the mail from Paul Anderson from Nov 21st, 2007 on the user list:
"Also, when using the SSO IFrame portlet, changing the password with Edit
after you Save your username for a remote system works fine.
But if you make a mistake and save the wrong username, you can't then
change the username (updateCredentialsForSite). Only the admin can do
this, by deleting the J2 user entry from the remote site with the SSO