“In 1784, five years before he became president of the United States, George Washington, 52, was nearly toothless. So he hired a dentist to transplant nine teeth into his jaw—having extracted them from the mouths of his slaves.”—The sorry legacy of the founders
What you write first is a first draft. When you get to the end of it, you’ve got a first draft. Anything can happen after that.
(In the first draft of Roald Dahl’s Charlie and the Chocolate Factory, while Charlie was touring Wonka’s factory, he was accidentally encased in chocolate and sent to Wonka’s house, where he foiled a burglary.)
There’s nothing to stop you fixing anything as you go, but it’s wiser to get to the end first, and then look at what you’ve got.
The book in your mind will pretty much always be better than the one that hits the paper. That’s just how it works.