The longer your story or series is, the more likely that one or more of your main characters is going to change significantly. And as far as realism is concerned, that’s good. People change based on their experiences, so changing is a big part of making characters feel more realistic. Plus, it’s useful if you want to keep your readers guessing by breaking your promises without really breaking them. The only problem with major character changes is how to do it without losing all of your fans.
For a lot of books, the main reason we read them is because we like the main characters. That means that if the main characters change in a way we don’t like, we can stop wanting to read the books. On the other hand, if the characters stay the same in book after book (not learning from their mistakes, for instance), then that can lose readers, too.
Picky, picky, picky.
So how do you change characters enough to make them feel real without losing readers? Well, you could study long-running series and see how they did it (Friends, The Simpsons, Naruto, the Harry Potter series, the Star Wars book series, etc.). You might find that some of the changes in those series did in fact lose fans; however, they still had a big enough fan base to keep going without worrying about being able to sell the story (they had to do something right, right?).
But if you don’t feel like researching on you’re own, you can go by my observations and hope that I’m right.
1. Keep the core principles.
If the character believes in something with all his/her heart, it’s going to take a helluva lot to change that. So for almost all characters, the main things they believe should stay the same. If they don’t, there needs to be a really good reason.
2. Make it gradual.
Even real people don’t change dramatically overnight unless they have some sort of come-to-God moment. See Sergeant York for an example of that – living through getting struck by lighting results in sudden strong religious beliefs, but even then, only 1 particular belief changed that happened to affect multiple behaviors. Not every aspect of the character changed quickly, and he had to fight old habits that he no longer believed in.
So most characters should change gradually, and even if a character suddenly decides to change something, actually making the change happen is likely to be a long, hard process.
3. Know what the character adds to the series. If the character spent the first few books being comic relief, making the character more serious might leave a void. You can do it, but something else needs to fill in for the lost humor if you don’t want the tone and style of the series to change. Or the character could show a serious side while still being a smart aleck.
The nice thing about long-term series is that they give 2D characters a chance to grow more depth, but be wary of completely losing the characteristics that gave them appeal in the first place.
That’s all I can think of right now, but there have to be other tricks to changing characters without losing all of your fans. If you can think of any, please, share. Can you think of any series that changed characters really well? Or ones that tried to and failed?