Neg_Utils/Editor/MonoBehaviourExtensions.cs

using System.IO;
using UnityEditor;
using UnityEngine;

namespace NEG.Utils.Editor
{
    public static class MonoBehaviourExtensions
    {
        [MenuItem("CONTEXT/MonoBehaviour/Change Script")]
        public static void ChangeScript(MenuCommand command)
        {
            if (command.context == null)
                return;

            var monoBehaviour = command.context as MonoBehaviour;
            var monoScript = MonoScript.FromMonoBehaviour(monoBehaviour);

            string scriptPath = AssetDatabase.GetAssetPath(monoScript);
            string directoryPath = new FileInfo(scriptPath).Directory?.FullName;

            // Allow the user to select which script to replace with
            string newScriptPath = EditorUtility.OpenFilePanel("Select replacement script", directoryPath, "cs");

            // Don't log anything if they cancelled the window
            if (string.IsNullOrEmpty(newScriptPath)) return;

            // Load the selected asset
            string relativePath = "Assets\\" + Path.GetRelativePath(Application.dataPath, newScriptPath);
            var chosenTextAsset = AssetDatabase.LoadAssetAtPath<TextAsset>(relativePath);

            if (chosenTextAsset == null)
            {
                Debug.LogWarning($"Selected script couldn't be loaded ({relativePath})");
                return;
            }

            Undo.RegisterCompleteObjectUndo(command.context, "Changing component script");

            var so = new SerializedObject(monoBehaviour);
            var scriptProperty = so.FindProperty("m_Script");
            so.Update();
            scriptProperty.objectReferenceValue = chosenTextAsset;
            so.ApplyModifiedProperties();
        }
    }
}