Preserve startdate on membership level change

preserve-startdate-including-registration-date.php

/*
	Keep members start dates at checkout, even if switching levels.
*/
function custom_pmpro_checkout_start_date($date, $user_id, $level) 
{
	global $wpdb;
			
	// Comment this out if you want to use the users registration date (Warning: Registration date may not contain data...)
	$sqlQuery = "SELECT UNIX_TIMESTAMP(startdate) FROM $wpdb->pmpro_memberships_users WHERE user_id = '" . esc_sql($user_id) . "' ORDER BY id LIMIT 1";		
	
	// Uncomment to use the user registration date rather than the "initial membership purchased" date (even if it's a "free" membership)
	// $sqlQuery = "SELECT UNIX_TIMESTAMP(user_registered) FROM $wpdb->users WHERE user_id = '" . esc_sql($user_id) . "' ORDER BY id LIMIT 1";		
	
	$old_timestamp = $wpdb->get_var($sqlQuery);
	
	if(!empty($old_timestamp))
		$date = date("Y-m-d H:i:s", $old_timestamp);
		
	return $date;
}
add_action('pmpro_checkout_start_date','custom_pmpro_checkout_start_date', 15, 3);