I didn't realize the modern concept of the soul says that it's unchanging. As you pointed out, that idea is a bit too fatalistic. Further, it implies it is a "thing", solid and substantial, which it clearly is not.

Some schools of Buddhism have a Soul-like mechanism in the Very Subtle Mind, which is the recorder and depository for Karma. This "soul", if you want to call it that, is changing all the time. Each thought, feeling, action is recorded as Karma, and this is postulated to survive death. It contains none of our memories, experiences, or "qualia" from our previous lives, but it contains the imprints on our Karma. Without this mechanism, there is no way for Karma to follow you from one incarnation to the next. The Very Subtle Mind is like a punch card recording your every activity and reaction. I guess Santa Claus must know how to read this punch card to determine who's been naughty and who's been nice.

Where the movie is very Buddhist is in the idea of exchanging self with others and abandoning self-cherishing as the key to enlightenment and pure and everlasting happiness, Nirvana.

Joe's behavior is entirely driven by self-importance due to his self-cherishing. In that regard, his thinking should be easily recognizable by 99.9999% of humanity.