How to change game object's parent in hierarchy without changing it's transform's position?

Here are a couple of scripts to add this functionality in Edit mode. In the Editor folder, add the script ObjectMoverInspector. You can place the script ObjectMover anywhere (in a folder called Helpers for example). Yes, I have awesome naming conventions =]

If the new menu item doesn't appear, just reopen the project. Then you can select Custom > Object Mover. This creates an empty object in the scene with the script ObjectMover attached. Drag and drop your child and new parent object in the Inspector then click the button.

ObjectMoverInspector.js

 #pragma strict
 
 @CustomEditor( ObjectMover )
 
 
 class ObjectMoverInspector extends Editor 
 {
     
     var childToMove : SerializedProperty;
     var newParent : SerializedProperty;
     
     var obj : GameObject;
     var objScript : ObjectMover;
     
     
     function OnEnable() 
     {
         obj = Selection.activeGameObject;
         objScript = obj.GetComponent( ObjectMover );
         
         childToMove = serializedObject.FindProperty( "childToMove" );
         newParent = serializedObject.FindProperty( "newParent" );
     }
     
     
     function OnInspectorGUI() 
     {
         // Update the serializedProperty - always do this in the beginning of OnInspectorGUI.
         serializedObject.Update();
         
         
         // childToMove
         EditorGUILayout.BeginHorizontal();
         EditorGUILayout.PropertyField( childToMove );
         EditorGUILayout.EndHorizontal();
         
         // newParent
         EditorGUILayout.BeginHorizontal();
         EditorGUILayout.PropertyField( newParent );
         EditorGUILayout.EndHorizontal();
         
         
         // Editor GUI Buttons
         EditorGUILayout.BeginHorizontal();
         
         if ( GUILayout.Button( "Reparent and position", GUILayout.MinWidth( 80 ), GUILayout.MaxWidth( 250 ) ) )
         {
             objScript.ChangeParentAndReposition();
         }
         
         EditorGUILayout.EndHorizontal();
         
         
         // Apply changes to the serializedProperty - always do this in the end of OnInspectorGUI.
         serializedObject.ApplyModifiedProperties();
     }
 }
 
 
 @MenuItem( "Custom/Object Mover" )
 static function CustomObjectMover() 
 {
     var go : GameObject = new GameObject( "_ObjectMover" );
     go.transform.position = Vector3.zero;
     go.transform.rotation = Quaternion.identity;
     go.AddComponent( ObjectMover );
 }

ObjectMover.js

 #pragma strict
 
 public var childToMove : Transform;
 public var newParent : Transform;
 
 private var localPos : Vector3;
 private var localRot : Quaternion;
 
 
 function ChangeParentAndReposition() 
 {
     if ( !childToMove || !newParent )
     {
         Debug.LogWarning( "Assign Objects in Inspector" );
         return;
     }
     
     Debug.Log( "Change Parent And Reposition" );
     
     localPos = childToMove.localPosition;
     localRot = childToMove.localRotation;
     
     childToMove.parent = newParent;
     childToMove.localPosition = localPos;
     childToMove.localRotation = localRot;
 }