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.