Fading between skyboxes

I'm currently working on a starfox-style game. The idea is for the player to play through all levels without going back to a menu. Thus I want to swap out the skybox and lighting at certain points of the game. I've managed to quite simply swap the skybox thus far, but what I really want is to have a smooth fading effect.

My code thus far looks like:

 using UnityEngine;
 using System.Collections;
 
 public class NewLevel : MonoBehaviour {
     public Material material1;
     public Material material2;
     public float duration = 2.0F;
     private bool newlevel= false;
     void Start() {
     newlevel = false;
     }
     void OnTriggerEnter(Collider other)
     {
         newlevel = true;
     }
     void Update() {
         if (newlevel = false) {
             RenderSettings.skybox= material1;
                 }
         if (newlevel = true) {
             float lerp = Mathf.PingPong (Time.time, duration) / duration;
             RenderSettings.skybox.Lerp (material1, material2, lerp);
             Debug.Log ("Skybox should be changing right about now");
         }
     }
 }

The thing is that thus far it does nothing. In fact, as soon as I start the game, the debug message pops up, despite nothing actually colliding with the box. Any suggestions?