Update post template for posts matching a certain criteria

update-post-template.php

<?php

$args = [
	'post_type' => 'post',
	'posts_per_page' => -1,
	'post_status' => 'any',
	'meta_query' => [
		[
			'key' => 'hide_thumbnail',
			'compare' => 'EXISTS'
		]
	]
];

$posts = get_posts($args);

foreach ($posts as $post) {

	if ((int) get_field('hide_thumbnail', $post->ID) !== 1) {
		continue;
	}

	update_post_meta($post->ID, '_wp_page_template', 'single-no-thumbnail');
}