Not familiar with the UpdateAdministrator function. Could you try SetAdminAclEntry instead? That should work for updating the Admin entry in the AMT device.
What is the error code you are receiving when you change the admin name? Please make sure that the new name conforms to the following rules -
Username for access control. Contains 7-bit ASCII characters, in the
range of 33-126, excluding :, ,, <, >, &, and characters. String
length is limited to 16 characters. Username cannot be an empty