Reimported fbx isn't updating new objects

Hi all, I have a problem creating and then subsequently editing fbx models in an external application (maya). After I drag and drop the fbx into the unity scene, it seems to become unchangeable. If I go back into Maya and make any changes to the fbx, I'd expect those changes to update in the untiy scene, but they don't, even though the fbx itself (in the assets folder) is changed.Can anyone help? Here is the workflow.

1. I have a hierarchy built in maya that I have imported into my Unity scene.

2. I change the fbx in Maya and save it over the fbx in the Unity projects assets folder.

3. When I go back into Unity the fbx in the assets directory has undated. (When I click on it the preview display has all the new objects). But the version in the scene/prefab still looks like the old one.

How do I get the object in the scene/prefab to reflect the changes made to the fbx file?

I've found this thread, which seems to describe the same problem, but I don't really get the answer.

http://answers.unity3d.com/questions/489035/reimport-fbx-with-new-objects.html

I'd rather not have to delete the old version from the scene and reimport the fbx from scratch.