Hair can naturally change from curly to straight when you hit puberty.
My nephew used to have really curly hair when he was a toddler but when he hit puberty the curly hair changed to straight hair.
So sometimes yes curly hair may change to straight on it's own due to changes in the body but if you've already hit puberty and your hair is still curly then it will likely not change to straight on it's own.
You can straighten your natural curly hair out though if you don't like the curly hair if it doesn't straighten out on it's own.