SPFolder movedFolder = oWeb.GetFolder("FolderPath");
                            foreach (SPFile file in movedFolder.Files) {
                                file.CheckOut();
                                file["ListID"] = NewItemID;
                                file.Update();
                                file.CheckIn("Updated", SPCheckinType.MajorCheckIn);

                            }

SPListItem listItem = file.Item;
    listItem["ListID"] = listID;
    listItem.Update();