You would basically have to loop through each of your posts:
$posts = get_posts();
foreach ( $posts as $post ) {
}
get_posts: https://codex.wordpress.org/Template_Tags/get_posts
Then in there, update the post meta to turn sections on:
update_post_meta( $post->ID, '_generate_use_sections', array( 'use_sections' => 'true' ) );
Totally untested, but theoretically that should turn Sections on within each page you looped through.
The hard part is taking your content from the default editor, and adding it into the first section. I don’t know how to do that off the top of my head, would take quite a bit of trial and error to figure out.