When using AD Bulk Users to update existing Active Directory users the program uses the sAMAcountName (username) to locate the user.
To update the sAMAccountName we need to change the attribute we use to search AD, in AD Bulk Users this is called the ID Column. Changing the ID Column to another unique attribute such as userPrincipalName or mail allows us to update the sAMAccountName. Below is an example CSV/Excel file.
Change the ID Column by clicking Options on the Main ribbon.