Changing the username of a super-admin of a multisite network appears to remove the "super" status, meaning that access to the network dashboard is blocked.
Workaround: (you are starting as "superadmin")
create "newuser", a new user.
give "newuser" super-admin status
Install this plugin
change "superadmin" to ""changedadmin" using the option in the dashboard of the main blog.
Log out - this was tricky, actually.
Log in as "newuser"
give "changedadmin" back the super-admin status.
delete "newuser" or better still keep it in reserve!
Hope this helps!