Preserve initial startdate for member regardless of how the membership level is updated.

pmpro_preserve_startdate.php

<?php
/*
Plugin Name: PMPro Preserve Startdate
Plugin URI: https://eighty20results.com/paid-memberships-pro/do-it-for-me/
Description: Customization to preserve initial membership signup (startdate) value.
Version: 1.0
Author: Thomas Sjolshagen @ Stranger Studios <thomas@eighty20results.com>
Author URI: https://eighty20results.com/thomas-sjolshagen/
*/
/*
	Keep members start dates at checkout, even if switching levels.
*/
function custom_pmpro_checkout_start_date($date, $user_id, $level)
{
    $date = custom_get_initial_date($user_id);
    return $date;
}
add_action('pmpro_checkout_start_date', 'custom_pmpro_checkout_start_date', 15, 3);

function custom_pmpro_updated_level()
{
    global $wpdb;

    $user_id = isset($_POST['user_id']) ? intval($_POST['user_id']) : null;
    $level_id = isset($_POST['membership_level']) ? intval($_POST['membership_level']) : null;

    // grab user info
    if (! is_null($user_id)) {
        $user = get_user_by('id', $user_id);
    }

    $startdate = custom_get_initial_date($user_id);

    $sql = $wpdb->prepare(
        "SELECT id
          FROM {$wpdb->pmpro_memberships_users}
          WHERE user_id = %d AND membership_id = %d
          ORDER BY id DESC
          LIMIT 1", $user_id, $level_id
    );

    $id = $wpdb->get_var($sql);

    if (WP_DEBUG)
        error_log("Found most recent DB record for {$user_id} in level {$level_id}: {$id}");

    if (! empty($id)) {
	    $wpdb->update( 
		    $wpdb->pmpro_memberships_users, 
		    array( 'startdate' => $startdate ), 
		    array( 'user_id' => $user_id, 'id' => $id ),
		    array( '%s' ),
		    array( '%d', '%d' )
	    );
    }
}
add_action('edit_user_profile_update', 'custom_pmpro_updated_level', 15);
add_action('personal_options_update', 'custom_pmpro_updated_level', 15);

function custom_get_initial_date($user_id) {

    global $wpdb;
    $date = null;
    
    $sqlQuery = $wpdb->prepare("
      SELECT startdate
        FROM {$wpdb->pmpro_memberships_users} AS mu
        WHERE mu.user_id = %d
        ORDER BY mu.id
        LIMIT 1
        ", $user_id
    );

    $old_start = $wpdb->get_var($sqlQuery);

    if (!empty($old_start)) {
	$old_timestamp = strtotime( $old_start, current_time( 'timestamp' ) );
        $date = date("Y-m-d H:i:s", $old_timestamp);
    } else {
    	$date = date("Y-m-d H:i:s", current_time('timestamp'));
    }

    return $date;
}