let’s say the user logged in under the username of ‘Simpson’, he’ll be asked to type his username in the ‘Your username’ field again, and then again, he can type anything, like ‘Roger23’ !
Simply just hide the username input if it’s not empty. I did that to avoid updating. Though i have other problems as you know 
Just put in the following:
if ($w("userinput").value === "") {
$w("userinput").show();
$w("profilebutton").hide();
$w("updatebutton").show();
}
else {
$w("userinput").hide();
$w("profilebutton").show();
//if you completely want to disable updating once a username is created
$w("updatebutton").hide();
}
}