fm-to-acf.php

<?php 
function convert_fm_to_acf() {
	$args = array(
		'posts_per_page'   => -1,
		'post_type'        => 'post',
	);
	$the_query = new WP_Query( $args );

	while ( $the_query->have_posts() ) {
		$the_query->the_post();
		$old_t = get_post_meta( get_the_ID(), 'wpp_episode_transcript', true );
		$old_s = get_post_meta( get_the_ID(), 'wpp_episode_sponsor', true );

		update_field( 'hibi_episode_sponsor', $old_s, get_the_ID() );
		update_field( 'hibi_transcript', $old_t, get_the_ID() );
	}

	wp_reset_postdata();
}

add_action( 'admin_init', 'convert_fm_to_acf' );