He could have re-staged the famous debate about miracles in which G.E.M Anscombe ripped holes in Lewis's argument, to the extent that he never wrote another book of theological argument. I didn't see Chaplin . Maybe they played down the fact that he was a comedian. I could go on, and on. In real life, Lewis reports Joy's last words were 'I am at peace with God'.
Film cuts it.
As in all previous versions of the story, we end with Lewis talking to Douglas Gresham, and the old man and the little boy crying together. Lewis, on this view, was emotionally frozen, and, in particular, couldn't bare to show emotion in front of other men. Hence, in the play, being able to embrace his stepson and weep was the culmination of a process which started when he met Joy. It is an extraordinarily affecting scene. And no big thing for him to say 'I believe in heaven' when we haven't been shown any significant loss of faith.