ng-change get new value and original value
With an angular {{expression}} you can add the old user or user.id value to the ng-change attribute as a literal string:
<select ng-change="updateValue(user, '{{user.id}}')" ng-model="user.id" ng-options="user.id as user.name for user in users"></select>On ngChange, the 1st argument to updateValue will be the new user value, the 2nd argument will be the literal that was formed when the select-tag was last updated by angular, with the old user.id value.
Also you can use
<select ng-change="updateValue(user, oldValue)" ng-init="oldValue=0" ng-focus="oldValue=user.id" ng-model="user.id" ng-options="user.id as user.name for user in users"></select>
Just keep a currentValue variable in your controller that you update on every change. You can then compare that to the new value every time before you update it.'
The idea of using a watch is good as well, but I think a simple variable is the simplest and most logical solution.