How to echange a script of a component?

Create yourself a editor script like t$$anonymous$$s. Given the fact that t$$anonymous$$s is a one-off script, I would not bother adding more usability - if you need to change types, edit the script before pus$$anonymous$$ng the button.

 using System;
 using UnityEditor;
 using UnityEngine;
 
 public class ReplacementScript: EditorWindow
 {
   public ReplacementScript ()
   {
   }
 
   [MenuItem("Window/Replace Magic")]
   public static void MenuOption ()
   {
     ReplacementScript window = (ReplacementScript)GetWindow (typeof(ReplacementScript));
     window.autoRepaintOnSceneChange = true;
     window.title = "BitGUI Toolbox";
     window.Show ();
   }
 
   public void OnGUI()
   {
     if (GUI.Button(new Rect(10, 10, 300, 20), "Push Me!"))
     {
       GameObject[] gos = GameObject.FindObjectsOfType(typeof (BaseBehaviour));
       foreach (GameObject go in gos)
       {
         BaseBehaviour oldObject = go.GetComponent<BaseBehaviour>();
         DerivedBehaviour newObject = go.AddComponent<DerivedBehaviour>();
 
         // copy all properties now
         // ...
 
         GameObject.DestroyImmediate(oldObject);
       }
     }
   }
 }

Thanks for your reply. The posted code snippet would be an adequate solution if there wouldn't be multiple inheritence levels and a lot of fields w$$anonymous$$ch have to be copied. Isn't there any better way? The proposed solution would mean to write a realy complex (de)serializer for components using relfection.