Alter post object before display

get_the_content() doesn’t actually return the post_content property from the global $post object. Instead it returns the first page of the global variable $pages, which is set before the_post hook is fired. See L287 of wp-includes/post-template.

To reset the $pages global, you can use the setup_postdata() method from the WP_Query object passed from the the_post hook.

add_action( 'the_post', 'wpse_the_post', 10, 2 );
function wpse_the_post( $post, $query ) {
  $post->post_title="Foo";
  $post->post_content="Yolo";

  //* To prevent infinite loop
  remove_action( 'the_post', 'wpse_the_post' );
  $query->setup_postdata( $post );
  add_action( 'the_post', 'wpse_the_post', 10, 2 );
}

This is kind of backwards since you’re then setting up the post data twice. But if you’re intent on using the_post hook…

I’d use two different filters instead (the_title and the_content).