Need to save nested prefab instrance as original prefab

Hi all

I have a situation when I have one prefab nested in the other prefab. And I strongly need to save changes made in the nested prefab to the original prefab. For example
Prefab A: floor plate (sample)
Prefab B: spawner of the procedural objects over the host object
As you can see, prefab B will be used as a nested prefab with many different prefab surfaces: steel floor plates, concrete, wood, walls, or even ceils.
I want to add prefab B as nested into prefab A, configure it, save, and then -- add it to many other construction prefabs.
But I can't save nested prefab to original! Any changes made in the nested prefab will remain only in the nested prefab.
I tried a simple script for calling from the customs inspector when I configure nested prefab.

public static void save_prefab_ex ( GameObject obj ){
GameObject prefab = PrefabUtility.GetCorrespondingObjectFromOriginalSource ( obj );
string prefab_path = AssetDatabase.GetAssetPath ( prefab );
bool success;
PrefabUtility.SaveAsPrefabAsset ( obj, prefab_path, out success );
}

But it keeps return error:
ArgumentException: Can't save part of a Prefab instance as a Prefab

How can I handle it?

P.S. This code useless as well

GameObject prefab = PrefabUtility.GetCorrespondingObjectFromOriginalSource ( r.gameObject );
string prefab_path = AssetDatabase.GetAssetPath ( prefab );
PrefabUtility.ApplyObjectOverride( r.gameObject, prefab_path, InteractionMode.AutomatedAction );

 

Figured out how to do it manually.
1. Open prefab asset for selected Prefab Instance A in the scene
2. Select nested prefab B
3. Override changes made in nested prefab B so they will appear in the original prefab B
But how to do it with script ?