first function migrates the simple id to acf relation array, second cleans the data

acf_meta_to_relation.php

<?php
function addMetaId2Relations($post_type, $meta_from, $meta_to){
    $arr = [
        'post_type' => $post_type,
        'posts_per_page' => -1,
        'fields' => 'ids'
    ];
    $q = new WP_Query($arr);
    echo 'poluted:<br><br>';
    foreach ($q->posts as $id) {
        $value_to_migrate = get_field($meta_from, $id, false);
        $relationValue = get_field($meta_to, $id, false);
        $relationValue[] = $value_to_migrate;
        update_field($meta_to, $relationValue, $id);
        echo $id.'<br>';
    }
}

function deleteMetaData($post_type, $meta_to_delete){
    $arr = [
        'post_type' => $post_type,
        'posts_per_page' => -1,
        'fields' => 'ids'
    ];
    $q = new WP_Query($arr);
    echo 'deleted:<br><br>';
    foreach ($q->posts as $id) {
        delete_field($meta_to_delete, $id);
        echo $id. '<br>';
    }
}