code: function change(x) {
    var u = this;
    var debug = u._upwardableDebug;
  
    if (x !== this.valueOf()) {
      u = make(x, { debug });
      getNotifier(this).notify({object: this, newValue: u, type: 'upward'});
  
      if (debug) {
        console.debug(...channel.debug("Replaced upwardable", this._upwardableId, "with", u._upwardableId));
      }
    }
    return u;
  }
docs: Change an upwardable. Issue notification that it has changed.