Expire post to draft by date-picker custom field

ACF saves the value as Ymd format in database regardless of display and return format. All you need is a meta query to fetch the correct posts.

function expire_posts_function() {
    
    //Get Expired Posts only
    $expired_posts = get_posts(arrasy(
        'post_type' => 'event',
        'posts_per_page' => -1,
        'post_status' => 'publish',
        'meta_query' => array(
            array(
                'key' => 'ev_date',
                'value' => date('Ymd'),
                'compare' => '<',
            ),
        ),
    ));
    
    //Loop through the posts and set status
    if ( $expired_posts ) {
        foreach( $expired_posts as $expired_post ) {
            wp_insert_post(array(
                'ID' => $expired_post->ID,
                'post_status' => 'draft',
            ));
        }
    }
}