AnimatorControllerOverride.clips[x].overrideClip is a Field, should be a property with only Get
http://docs.unity3d.com/ScriptReference/AnimatorOverrideController.html

Tried using AnimatorController from script. Set AnimatorController.clips[x].overrideClip to an animation, was surprised to see nothing changed.

Steps:
Run TestScene, press changeAnim button. See text results.
Check TryChangeAnim script for bugged code

Fixes;
Either make overrideClip a Get only property, or actually make setting it work.