https://forum.unity.com/threads/replace-game-object-with-prefab.24311/

PrefabReplace.cs

// COMMUNITY THREAD LINK https://forum.unity.com/threads/replace-game-object-with-prefab.24311/
// CopyComponents - by Michael L. Croswell for Colorado Game Coders, LLC
// March 2010
//Modified by Kristian Helle Jespersen
//June 2011
//Modified by Connor Cadellin McKee for Excamedia
//April 2015
//Modified by Fernando Medina (fermmmm)
//April 2015
//Modified by Julien Tonsuso (www.julientonsuso.com)
//July 2015
//Changed into editor window and added instant preview in scene view
//Modified by Alex Dovgodko
//June 2017
//Made changes to make things work with Unity 5.6.1
//March 2018
//Added link to community thread, booleans to chose if scale and rotation are applied, mark scene as dirty, changed menu item to tools. By Hyper
//May 2018
//Added KeepPlaceInHeirarchy self explanatory
//Modified by Virgil Iordan
//Added unique numbering identifier in the hierarchy to each newly instantiated prefab, also accounts for existing numbers
//Modified by Dev Bye-A-Jee, Sanjay Sen & Nick Rodriguez for Ravensbourne University London
//January 2019

using System;
using System.Collections;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using UnityEditor;
using UnityEngine;
using UnityEngine.SceneManagement;
using UnityEngine.UI;
 
public class PrefabReplace : EditorWindow
{
    [SerializeField] private GameObject prefab;
    private bool selectionChanged;
    private string objectsToSearch = "";
    private List<GameObject> foundObjects = new List<GameObject>();
    private GUIStyle guiStyle = new GUIStyle(); //create a new variable
    private int count = 0;
    private bool addFoundObjects;
    private bool keepNames = true;
 
    [MenuItem("Tools/Prefab Replace")]
    static void CreateReplaceWithPrefab()
    {
        int width = 340;
        int height = 370;
 
        int x = (Screen.currentResolution.width - width) / 2;
        int y = (Screen.currentResolution.height - height) / 2;
     
        GetWindow<PrefabReplace>().position = new Rect(x, y, width, height);
    }
 
    private void OnGUI()
    {
        guiStyle.fontSize = 15; //change the font size
        Searching();
        GUILayout.Space(10);
        Replacing();
        GUILayout.Space(50);
        Settings();
    }
 
    private void Searching()
    {
        //GUI.Label(new Rect(10, 15, 150, 20), "Search by name", guiStyle);
        objectsToSearch = GUI.TextField(new Rect(90, 35, 150, 20), objectsToSearch, 25);
 
        if (objectsToSearch != "")
        {
            GUI.enabled = true;
        }
        else
        {
            GUI.enabled = false;
            count = 0;
        }
        GUILayout.Space(15);
        if (GUILayout.Button("Search"))
        {
            foundObjects = new List<GameObject>();
            count = 0;
 
            foreach (GameObject gameObj in GameObject.FindObjectsOfType<GameObject>())
            {
                if (gameObj.name == objectsToSearch)//.Contains(objectsToSearch))// == objectsToSearch)
                {
                    count += 1;
                    foundObjects.Add(gameObj);
                    foreach (Transform child in gameObj.transform)
                    {
                        count += 1;
                        foundObjects.Add(child.gameObject);
                    }
                }
            }
 
            if(foundObjects.Count == 0)
            {
                count = 0;
            }
        }
 
        GUI.enabled = true;
        EditorGUI.LabelField(new Rect(90, 65, 210, 15), "Number of found objects and childs");
        GUI.TextField(new Rect(90, 80, 60, 15), count.ToString(), 25);
 
        GUILayout.Space(100);
        if (count > 0)
        {
            GUI.enabled = true;
        }
        else
        {
            GUI.enabled = false;
        }
        if (GUILayout.Button("Replace found objects"))
        {
            if (prefab != null)      
            {
                InstantiatePrefab(foundObjects);
            }
        }
 
        GUI.enabled = true;
    }
 
    private void Replacing()
    {
        GUILayout.Space(20);
        GUILayout.BeginVertical(GUI.skin.box);
        GUILayout.Label("Replacing");
        GUILayout.Space(20);
 
        prefab = (GameObject)EditorGUILayout.ObjectField("Prefab", prefab, typeof(GameObject), false);
 
        var selection = Selection.objects.OfType<GameObject>().ToList();
 
        if (selectionChanged)
        {
            if (selection.Count == 0)
                GUI.enabled = false;
 
            for (var i = selection.Count - 1; i >= 0; --i)
            {
                var selectedObject = selection[i];
                if (prefab != null && selection.Count > 0 &&
                    selectedObject.scene.name != null
                    && prefab != PrefabUtility
                    .GetCorrespondingObjectFromSource(selectedObject))
                {
                    GUI.enabled = true;
                }
                else
                {
                    GUI.enabled = false;
                }
            }
        }
        else
        {
            GUI.enabled = false;
        }
 
        if (GUILayout.Button("Replace"))
        {
            InstantiatePrefab(selection);
            selectionChanged = false;
        }
 
        GUILayout.Space(10);
        GUI.enabled = true;
        EditorGUILayout.LabelField("Selection count: " + Selection.objects.OfType<GameObject>().Count());
 
        GUILayout.EndVertical();
    }
 
    private void Settings()
    {
        keepNames = GUILayout.Toggle(keepNames, "Keep Names");
    }
 
    private void OnInspectorUpdate()
    {
        Repaint();
    }
 
    private void OnSelectionChange()
    {
        selectionChanged = true;
    }
 
    private void InstantiatePrefab(List<GameObject> selection)
    {
        if (prefab != null && selection.Count > 0)
        {
            for (var i = selection.Count - 1; i >= 0; --i)
            {
                var selected = selection[i];
                Component[] components = selected.GetComponents(typeof(MonoBehaviour));
                if (components.Length == 0)
                {
                    SceneManager.SetActiveScene(SceneManager.GetSceneByName(selected.scene.name));
 
                    var prefabType = PrefabUtility.GetPrefabAssetType(prefab);
                    GameObject newObject;
 
                    if (prefabType == PrefabAssetType.Regular)
                    {
                        newObject = (GameObject)PrefabUtility.InstantiatePrefab(prefab);
                    }
                    else
                    {
                        newObject = Instantiate(prefab);
 
                        if (keepNames == false)
                        {
                            newObject.name = prefab.name;
                        }
                    }
                    if (newObject == null)
                    {
                        Debug.LogError("Error instantiating prefab");
                        break;
                    }
 
                    Undo.RegisterCreatedObjectUndo(newObject, "Replace With Prefabs");
                    newObject.transform.parent = selected.transform.parent;
                    newObject.transform.localPosition = selected.transform.localPosition;
                    newObject.transform.localRotation = selected.transform.localRotation;
                    newObject.transform.localScale = selected.transform.localScale;
                    newObject.transform.SetSiblingIndex(selected.transform.GetSiblingIndex());
                    if (keepNames == true)
                    {
                        newObject.name = selected.name;
                    }
                    Undo.DestroyObjectImmediate(selected);
                }
            }
        }
    }
}