Copy old meta fields into new meta fields

functions.php

<?php
$args = array(
    'post_type' => 'events', // Adjust post type if needed
    'posts_per_page' => -1, // Retrieve all posts
);

$posts_query = new WP_Query($args);

if ($posts_query->have_posts()) {
    while ($posts_query->have_posts()) {
        $posts_query->the_post();
        $post_id = get_the_ID();

        // Get old general event date
        $startTime = get_post_meta($post_id, 'event_date', true);

	// Set it as new start date
        update_post_meta($post_id, 'event_date_start', $startTime);
    }

    // Restore original post data
    wp_reset_postdata();
}