UnityBulkReplacerTool as seen on http://blog.camposanto.com/post/92843727164/bulk-replacer-tool-time-for-another-short-code

UnityBulkReplaceTool.cs

using UnityEngine;
using UnityEditor;

using System.Collections;
using System.Collections.Generic;

public class UnityBulkReplaceTool : MonoBehaviour
{
	public GameObject objectToReplace = null;
	public GameObject objectToSpawn = null;

	[MenuItem("Window/Video Game/Bulk Replacer", false, vgMath.vgWindowPriority)]
	void Start()
	{
		// Get existing open window or if none, make a new one:
		EditorWindow.GetWindow(typeof(vgBulkReplaceWindow));
	}

	void OnGUI()
	{
		EditorGUILayout.BeginVertical();

		objectToReplace = (GameObject)EditorGUILayout.ObjectField("Object To Replace:", objectToReplace, typeof(GameObject), true);

		objectToSpawn = (GameObject)EditorGUILayout.ObjectField("Prefab To Spawn:", objectToSpawn, typeof(GameObject), true);

		if( objectToReplace != null )
		{
			GameObject[] selectedObjects = GameObject.FindObjectsOfType<GameObject>();

			List<GameObject> objectList = new List<GameObject>();

			for( int i = 0; i < selectedObjects.Length; ++i )
			{
				GameObject testObject = selectedObjects[i];
				if( testObject.name == objectToReplace.name )
				{
					objectList.Add(selectedObjects[i]);
				}
			}

			Selection.objects = objectList.ToArray();

			if( GUILayout.Button(new GUIContent("Replace All Objects", "Replaces all objects with selected prefab")) )
			{
				foreach( GameObject go in objectList )
				{
					GameObject newObject;
					newObject = PrefabUtility.InstantiatePrefab(objectToSpawn) as GameObject;

					if( newObject == null )
					{
						Debug.LogError("objectToSpawn is not a prefab!", newObject);
						break;
					}

					newObject.transform.position = go.transform.position;
					newObject.transform.rotation = go.transform.rotation;
					newObject.transform.parent = go.transform.parent;

					Undo.RegisterCreatedObjectUndo(newObject, "Bulk replaced");

					Undo.DestroyObjectImmediate(go);
				}
			}
		}
		else
		{
			GreyedOutButton("Replace All Objects", "Must select object to replace");
		}

		if( GUILayout.Button("Clear") )
		{
			objectToReplace = null;
			objectToSpawn = null;
		}

		EditorGUILayout.EndVertical();
	}
}