Lewis has this idea of a journey beyond death. It comes up again and again from The Problem of Pain to The Great Divorce and so on. In the eighth chapter of TGD Lewis adds this idea that you cannot exist in heaven for long without undergoing this change. There is a time line for change and when you refuse time and again to change, it becomes harder for you to remain in heaven. At this point in time the narrator has not yet realized that change is possible and starts to think that it is entirely possible that this place could be evil. It is interesting that Lewis mentions this because it combines his thoughts on our unworthiness of coming before the throne (even after death) and his idea that free will exists in heaven. It also goes to show that heaven cannot be embraced if we still hold onto any portion of this world, including ourselves. It is only when we stop worrying about ourselves that heaven becomes a reality.