Can you override a customeditor if it already exists?

Unity has some already defined custom inspectors, for example in the mecanim system. I’d like to be able to override some, but it seems that [CustomEditor(typeof(object))] only works the first time it is loaded. Is there any way to override one that is already defined?

For example:
[CustomEditor(typeof(Transition))] is already defined in UnityEditor.dll, so I’m trying to figure out how to have my own Transition inspector custom editor.

Confirmed workaround for this problem.

I used an EditorWindow for my tool which also monitors what current selected objects are. For this example, I’m editing a State object since the Transition object I mentioned in my first post is not exposed (internal class). The ‘StateEventEditor’ is my own customeditor class.

In the EditorWindow:

private static State lastSelectedState;

void OnEnable () {
    EditorApplication.update += Update;

void Update () {
    if( Selection.activeObject.GetType() == typeof(State) ) {
		if( Selection.activeObject != lastSelectedState )
			Debug.Log("State selected");
			lastSelectedState = (State)Selection.activeObject;
			StateEventEditor stateEventWrapper = new StateEventEditor(lastSelectedState);