No, don't try to change the material of sprites. Change the material of your atlas instead. Also, why bother with asset bundles at all?
using UnityEngine;
using System.Collections;
public class ReplaceAtlasMaterial : MonoBehaviour
{
public UIAtlas atlas;
public string url = "http://www.yourwebsite.com/atlas.png";
IEnumerator Start ()
{
yield return www;
atlas.spriteMaterial.mainTexture = www.texture;
www.Dispose();
}
}