gistfile1.cs

public class BulkMaterialSwapWindow : MonoBehaviour {
    public Material oldMat;
    public Material newMat;

    [MenuItem("WFTO/Bulk Asset Modifier/Bulk Material Swap")]
    static void Init() {
        BulkMaterialSwapWindow window = EditorWindow.GetWindowWithRect(typeof(BulkMaterialSwapWindow), new Rect(0, 0, 165, 100));
        window.Show();
    }

    void OnGUI() {
        EditorGUILayout.BeginHorizontal();
        oldMat = EditorGUILayout.ObjectField("Old Material", oldMat, typeof(Material), true);
        EditorGUILayout.EndHorizontal();
        EditorGUILayout.BeginHorizontal();
        newMat = EditorGUILayout.ObjectField("New Material", newMat, typeof(Material), true);
        EditorGUILayout.EndHorizontal();
        if (GUILayout.Button("Swap material"))
            if (oldMat == null || newMat == null)
                .ShowNotification(new GUIContent("No material selected for swapping!"));
            else
                SwapMaterials();
        
    }

    private void SwapMaterials() {
        var assets = GetAssetsOfType<Object>(".prefab");
        Debug.Log(assets.Length + " prefabs found.");
        int materialCounter = 0;

        foreach (var assetObject in assets) {
            GameObject assetGO = assetObject as GameObject;
            if (assetGO == null)
                continue;

            List<Renderer> renderers = new List<Renderer>();
            renderers.AddRange(assetGO.GetComponentsInChildren<Renderer>(true));

            foreach (var renderer in renderers) {
                materialCounter++;
                renderer = newMat;
            }
        }

        Debug.Log(materialCounter + " materials swapped.");
        AssetDatabase.SaveAssets();
        AssetDatabase.Refresh();
    }
}