CMB2 to ACF Repeater

functions.php

// make sure to comment out add_action when you're done, or working on the data
add_action( 'init', 'cmb2_to_acf' );

function cmb2_to_acf() {
  // You can use offset if needed.
	$args = array(
		'post_type'   => 'some_post_type',
		'numberposts' => 400, // whatever number.
		// 'offset'      => 800,
	);

	$my_posts = get_posts( $args );

	foreach ( $my_posts as $my_post ) {
		my_acf_save_post( $my_post->ID );
	}
}

function my_acf_save_post( $post_id ) {
	$meta = get_post_meta($post_id);
  // CMB2 data is serialized. Well, at least mine was ;)
	$cmb_array = unserialize($meta['_some_cmb2_group'][0]);
  	// I used the ACF Repeater keys here, but didn't need it for the subfield below.
	if( is_array( $cmb_array ) ) {
		$repeater_field_key = 'field_5da77d6327885';
		$title_key = 'field_5da77dfb27886';
		$button_key = 'field_5da78013c748f';
		$file_upload_key = 'field_5da77e0427887';
		$content_key = 'field_5da77e5c27888';
		$value_array = [];
		$counter = 0;
		foreach ( $cmb_array as $data ) {
			$value_array[ $counter ] = array(
				'section_title' 	 => $data['description'],
				'file_upload' 		 => $data['file_id'],
				'button_text' 		 => $data['file_text'],
				// Not sure if html_entity_decode is needed, but it worked:  https://stackoverflow.com/a/57491553/1843781
				'additional_content' => html_entity_decode($data['content']),
			);
			$counter++;
		}
		update_field( $repeater_field_key, $value_array, $post_id );
	} else {
		return;
	}

}