Convert file_list to CMB2 repeatable field groups. Go through post meta by a specified key. Convert file_list data to a CMB2 repeatable field group.

functions.php

<?php
/*
Plugin Name: Convert file_list to CMB2 repeatable field groups
Plugin URI: https://www.iweb.co.uk/
Description: Go through post meta by a specified key. Convert file_list data to a CMB2 repeatable field group.
Version: 1.0.0
Author: iWeb
Author URI: https://www.iweb.co.uk/
*/

if ( isset( $_GET['convert_file_list'] ) ) {
	add_action( 'template_redirect', function () {
		global $wpdb;

		$results = $wpdb->get_results(
			"
			SELECT pm.post_id, pm.meta_value FROM {$wpdb->postmeta} pm
        	LEFT JOIN {$wpdb->posts} p ON p.ID = pm.post_id
        	WHERE pm.meta_key = '_cmb_banners'
        	"
		);

		if ( empty( $results ) ) {
			die( 'No post meta with that key.' );
		}

		foreach ( $results as $result ) {
			printf( 'Working on post ID %s:<br>', $result->post_id );

			$meta_value = maybe_unserialize( $result->meta_value );

			$new_format = array();

			foreach ( $meta_value as $key => $value ) {
				$new_format[] = array(
					'image_id' => $key,
					'image'    => $value,
					'url'      => '',
					'caption'  => '',
				);
			}

			update_post_meta( $result->post_id, '_cmb_banners', $new_format, $meta_value );
		}

		die( 'Done.' );
	} );
}