How do I replace an asset for a prefab?

Questions & Answers
1

I have this scene with hundreds of copies of the same asset. How do I substitute them by a prefab? Keeping the same position and rotation is important.
Thx

2

Ok, two questions to clarify:

  1. Are any of these assets referenced somewhere else? In other words: would it be OK to automatically delete and recreate (at the same spot) all of these assets or will that cause a multitude of broken links?
  2. How do you know (automatically) what assets need to be replaced? E.g. do you mean “all assets with a specific name?” Or do they all have a specific tag? Or are they all assets that are childs of a specifc gameObject? Or all gameObjects that have a specifc component? Or maybe “just all assets that I have currently selected?”

I assume the answer to 1 is “broken links don’t matter - recreate is fine” and the answer to 2 is “all objects that I have currently selected”.

Then you could write an editor tool like this:

// put this in some editor scripte, i.e. in some C# file under "Editor/" 
// directory with "using UnityEditor;" included
[MenuItem("Helper/Replace Objects With Prefab")]
static void HelperReplaceObjectsWithPrefab()
{
    var prefab = AssetDatabase.LoadAssetAtPath<GameObject>("Assets/myPrefab.prefab"); // put the location of the prefab here

    // go over all selected objects. Change this if you want to go
    // through other objects, e.g. GameObject.FindWithTag ...
    foreach (GameObject s in Selection.objects)
    {
        // create a new prefab
        var go = (GameObject)PrefabUtility.InstantiatePrefab(prefab);
        go.name = s.name; // remember name
        EditorUtility.CopySerialized(s.transform, go.transform); // remember position/rotation etc..
        go.transform.parent = s.transform.parent; // put into hierarchy
        PrefabUtility.RecordPrefabInstancePropertyModifications(go); // fix modified properties
        GameObject.DestroyImmediate(s); // destroy old
    }
}

The code is 100% untested and written out of the blue, but I hope you get the idea ^^.