Values changed with Editor Script don't get saved

I have a simple editor script that alters the sortingOrder property of the SpriteRenderer of my GameObjects (which are mostly instantiated prefabs). It all works fine, however, after I saved my project and reopen it, it reverts all values back to what they were before I altered them.

Why is that and how can I make it persist?

     if(GUILayout.Button("Fix Z-Order")) {
         GameObject group = GameObject.Find("Doodads");
         if(group == null)
             Debug.LogError("GameObject not found!");
         SpriteRenderer[] srs = group.GetComponentsInChildren<SpriteRenderer>();
         foreach(SpriteRenderer r in srs) {
             float y = r.transform.position.y;
             r.sortingOrder = -(int)(y * 100);
         }
     }