Make your JQuery call more robust by adding success and error callbacks like this:
$('#ChangePermission').click(function() {
$.ajax({
url: 'change_permission.php',
type: 'POST',
data: {
'user': document.GetElementById("user").value,
'perm': document.GetElementById("perm").value
},
success: function(result) { //we got the response
alert('Successfully called');
},
error: function(jqxhr, status, exception) {
alert('Exception:', exception);
}
})
})