A function for converting existing WordPress meta keys to new values.

convert-post-meta.php

/**
 * Convert existing meta keys to new meta keys.
 *
 * @since x.x.x
 *
 * @global object $wpdb The current database.
 *
 * @param string $existing_key The existing meta key.
 * @param string $new_key The new meta key.
 * @param bool $remove_existing Optional. Whether to remove the old meta entries after conversion.
 * @return bool Whether any meta keys were converted.
 */
function convert_post_meta( $existing_key, $new_key, $remove_existing = false ) {
    
    global $wpdb;
    
    $existing_metas = $wpdb->get_results( $wpdb->prepare(
        "
        SELECT value.meta_value, value.post_id
        FROM $wpdb->postmeta value
		WHERE value.meta_key = '%s'
		",
        $existing_key
	), ARRAY_A );
	
    if ( ! empty( $existing_metas ) ) {
        
        foreach ( $existing_metas as $existing_meta ) {
            add_post_meta( $existing_meta['post_id'], $new_key, $existing_meta['meta_value'] );
        }
        
        if ( $remove_existing ) {
        	delete_post_meta( $existing_meta['post_id'], $existing_key );
        }
        
        return true;
        
    } else {
        return false;
    }
}