gistfile1.cs

public class BulkMaterialSwapWindow : MonoBehaviour {
    public Shader oldShader;
    public Shader newShader;

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

    void OnGUI() {
        EditorGUILayout.BeginHorizontal();
        oldShader = EditorGUILayout.ObjectField("Old Shader", oldShader, typeof(Shader), true);
        EditorGUILayout.EndHorizontal();
        EditorGUILayout.BeginHorizontal();
        newShader = EditorGUILayout.ObjectField("New Shader", newShader, typeof(Shader), true);
        EditorGUILayout.EndHorizontal();
        if (GUILayout.Button("Swap Shader"))
            if (oldShader == null || newShader == null)
                .ShowNotification(new GUIContent("No Shader added for swapping!"));
            else
                SwapShader();
        
    }

    private void SwapShader() {
        var assets = GetAssetsOfType<Object>(".mat");
        Debug.Log(assets.Length + " materials found.");
        int materialCounter = 0;

        foreach (var assetObject in assets) {
            Material assetMat = assetObject as Material;
            if (assetMat == null)
                continue;

            if(assetMat.shader == oldShader)
                assetMat.shader = newShader;
        }

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