One of my favorite philosophical paradoxes is the Ship of Theseus. If I have a ship and I gradually replace the parts of the ship as they break or suffer from wear and tear, it’s possible that eventually I will have a ship with none of the original parts. The real ship of Theseus — its physical components — are in a junkyard. Even if it still operates perfectly, is it still the Ship of Theseus? Does it still retain its specific identity though it is a completely different ship?
Though it may not be a physical transformation, our favorite fictional characters “switch out” some of their parts during and after the story’s conflict. Usually what is changed is something like their personality or the way they view the world or their belief system. The person they are at the conclusion isn’t (I would even say mustn’t) be the same person they were during the exposition. Through conflicts and the reaction to these conflicts the character transforms into something new.
Except for ongoing, installment fiction like comic books, there usually isn’t enough “storytime” to transform 100% of a character. There are some special cases in science fiction, as in the artificial body-swapping technology in the Ghost in the Shell films and manga series. Interestingly, one of the running themes in that universe is the search for human identity in the face of ultra-advanced technology. What does it mean to be a human when everything about you can be changed?
Knowing all of this, at what point are the characters with whom we identify — or the ones we don’t — aren’t the same character? They may be called the same name, have the same social security number, and take up the same amount of space as their original selves, but there may be a point at which the person becomes fundamentally and irreversibly different. How can a writer accomplish such a metamorphosis on their own “Ship of Theseus” realistically? Should they even attempt it at all?