how have your characters changed as you've written them?

You're definitely not alone in that! I've got 6 characters that I've been developing for about a year or so now. The first one is Grace. Her name started out being Meena. She was still a ghost but had more powers and was a generally happy person. Her name then changed to Julia as I developed her backstory more. Then, I heard a song called "Three Kings Found The Lord" and it inspired a rewrite of her character. She became lost soul's "Saving Grace", therefore her name is Grace. Kat came from two scrapped characters. One was a cyborg and the other a mechanic. I merged them together to make Kat, shortening her hair, changing clothing aspects, and making her more salty as time went on. Nova changed a ton. Her outfit mostly and also a bit of her origin story. Everyone needs a family right? She got one! Elvira stayed a skeleton for all of her development but some of who she is changed. Lastly, Kiara and Bailey who didn't change much besides becoming more believable and having more interesting elements to them.
I believe character development is not only the hardest, but the most important thing in the writing process. Don't be afraid to let things go and explore other ideas. If anyone reading this is struggling, just remember the question "why?" Asking "why" helps give characters a reason for what they do. It helps believability. That's all I have to say, bye!

Hoo boi. I have one minor character who started out as a shy boy named Ethan who served as a love interest to a main character. Then that role was given to a different character and Ethan become a hot shot asshole pilot who didn’t know how to keep his mouth shut and was a general nuisance. He just wasn’t doing anything for the story. So I took him all the way back to square one, made him a girl, and tweaked her personality and made her more relevant to the story.
There’s also an agender alien character who started out as a twitchy newbie ambassador, and was then uprooted from their story’s universe, plopped into a different one. They then became an anthropologist who was transferred to a secret experiment regarding an uncooperative human subject.
Thalia went from a stuffy nerd botanist in her mid-thirties to a quirky (and still nerdy) 19 year old with a fierce temperament.

I'm going to jump in this.

I started out with an idea for a character that can see emotions. His name, for now, will be emo-boy. Fleshing out his universe was hard since I had another universe far more fleshed out. March or April of last year I asked for suggestions for various names. I don't remember many, only Cedar and Jay. I chose Cedar.
I'm a lazy person, and making a whole new universe from scratch for one character I didn't have anything but a name and an emo personality did not sound appealing to me. I merged him with my other universe, which at the time was named Usotm. I made him the child of two of my main characters, Audra and James. Now I had a set design for him since James is mixed European and Audra is half Asian. Their magical abilities worked with his powers, which I called "Emo- Clairvoyance".
My first draft for him was… cringy. He was kid-like and very "The world hates me… I see everyone else's happiness but yet I can't have any…" I gave him a sister who was also very kid-like.
Not satisfied with my results, I took a break from dealing with Cedar for a few months until I started worldbuilding in a different section of the same universe. I had renamed it to "TUO", "The Untold Obsurities".
My writing abilities grew, going from 6-7th-grade level to 11-12th. When this happened it became apparent what I could do with him and my growing world.
He went from aro-ace, to straight, to gay. I made him 17 and have a more mature outlook on the world. I gave him less of an emo stance and made him macho and a deviant. I fleshed out his powers, renaming them to "Emotional perceruption" (Emotional perception-eruption). I turned the useless ability to see emotions into a stealth to war weapon. The 14yo emo-boy became a mafia leader rebel going against the government.
Here's his current character page because I tend to become a huge word salad. XD Cedar Paul Rullond

I have literally over 100 ocs, but the first one I made when I was six. Her name is Swift. She was originally 16, had Neko ears, super girly, had long hair, and wore this gold dress with a pink ribbon sash (and no shoes). Since I created her 12 years ago, she has lost her left arm, her tongue, cut her hair down to a pixie cut, had two kids, aged 16 years, been married and divorced. Her style is much more tomboyish. She has prosthetics both for the tongue and the arm. I've also made her way less Mary-Sue (I made her when I was six so of course she had no flaws) and now she's a little bit of a jerk, but tries not to be. She used to be a reflection of me (a self insert), but now she's my polar opposite. Oh, and she's not a furry anymore, but I did give her super speed because it tied well with her story. I'm sure there's more I'm missing, but… for now, I think that's it?
EDIT: I forgot that I turned her from straight to pansexual and polyamorous. Her now ex-wife transitioned into a woman, and their shared partner was a man. All 3 ocs are super old, Alex (her girlfriend) I also made when I was six to be a boyfriend of Swift (literally no personality) and Cody (their ex-lover and now Alex's husband) I made when I was 11 or 12.