Dont really understand your question. Why don’t you want to change the username? Your question isn’t properly written. Could you be more specific?
IMHO, it is the task of the programmer to securely program the application to handle those modifications on the server. In my CMSs for example, I double check the username against the database and make them unique, the validation of the username against the DB with their correspondent passwords and salt values will find out you are doing shifty business.
If you modify the username on the client and I have just dropped the details of user: peter instead of admin, I will immediately detect if you have changed the username and give you a nice message or get you out of the application with a nice report sent to the administrator.