Thanks. That got me on the right track. If this can help anyone, here is how I did it. After I used phpmhyadmin to remove the fields I didn’t need. I didn’t concern myself with the excerpt because wp auto generates it.
$args = array( 'post_type' => 'custom_post_name', 'posts_per_page' => -1 );
$loop = new \WP_Query( $args );
$count = 1;
while ( $loop->have_posts() ) : $loop->the_post();
$legacy_content = get_field( 'legacy_content' );
$content = get_the_content();
// If custom content field is not empty and wordpress native content is empty
if ( !$legacy_content == '' && $content == '' ) {
$post_args = array(
'ID' => get_the_ID(),
'post_content' => $legacy_content, // update legacy content field to post_content
);
wp_update_post( $post_args );
update_field( 'legacy_content', '', get_the_ID() );
update_field( 'legacy_excerpt', '', get_the_ID() );
}
endwhile;