Convert cloneable values

convert.php

<?php
function prefix_convert( $field_id ) {
    $query = new WP_Query( [
        'post_type'      => 'event',
        'posts_per_page' => -1,
    ] );
    if ( ! $query->have_posts() ) {
        return;
    }
    while ( $query->have_posts() ) {
        $query->the_post();
        $values = rwmb_meta( $field_id );
        if ( ! is_array( $values ) || empty( $values ) ) {
            continue;
        }
        delete_post_meta( get_the_ID(), $field_id );
        foreach ( $values as $value ) {
            add_post_meta( get_the_ID(), $field_id, $value );
        }
    }
    wp_reset_postdata();
}
 
add_action( 'init', function() {
    if ( ! isset( $_GET['unique_key'] ) ) {
        return;
    }
    $field_id = 'start_date';
    prefix_convert( $field_id );
} );