Server settings are accessible from the menu in the login window. Type your new user name, go to server settings and change them, close server settings and press Login.
MT and MSN Spaces use separate passwords for web and downloadable clients. For MSN you should enter your journal name (i.e. what is after spaces.msn.com/members/) as username and secret word from email posting page instead of password.
Update Apr 2007: old version of Atom does not work with new Blogger. Enter your Google account email and password instead of username and password for Blogger account.