I am in the process of migrating posts from an existing wp site into a new wp site. The existing site has a custom field called subtitle. When I import the blogs into the new site, I’d like populate my simple acf subtitle text field with these hand coded custom fields.

What is the best way to do this?

Here is the code I’ve found in the existing sites single.php that appears to be creating the sub title field:

$mt_post_subtitle = get_post_meta($post->ID, "mt_post_subtitle", true);
<?php echo esc_attr($mt_post_subtitle); ?>

Please let me know if I’m missing something and let me know what I can add to help solve this. Thanks in advance.