The first draft never looks much like the final one.
This is a fact I am learning.
And its okay. It's okay that the characters change. Their pasts change. The defining decisions in their lives change.
It's okay that the novel you outlined however long ago doesn't look anything like the novel you just finished. And it will barely resemble the after-editing manuscript you present to agents one day. And that's okay too.
I have found that the changes don't take away from the book. Usually they make it more interesting.
For example, my latest heroine has lots of tattoos. When I started writing the book, her tattoos were just a part of her exterior. Half-way through the manuscript I realized they needed a purpose. A girl like this one wouldn't just have lots of colorful tattoos. She would have them for a reason. Something deep. Possibly something painful.
So half-way through my novel, I learned something new about the heroine.
Your characters should evolve. They should grow. And the arc of your story should change. With each book you will grow as a storyteller and your characters will grow in depth. This is a good thing. Go with it.