Change how codes are generated for orders in Paid Memberships Pro. Add this code to your active theme's functions.php or a custom plugin.

pmpro_order_codes.php

/*
    Set random order "code" to equal the order ID
	
	It is important to keep the prefix (PMPRO-) below or change it to something else with a non-number in it. 
    If a code is all numeric, PMPro will go into an infinite loop when a new order is created.
*/
function my_pmpro_random_code($code, $order)
{
	global $wpdb;

	$prefix = 'PMPRO-';

	//already have an id for this order? or should we guess what the next id is going to be?
	if(!empty($order->id))
		$code = $prefix . $order->id;
	else {
		//get id of most recent order
		$last_order_id = intval($wpdb->get_var('SELECT id FROM $wpdb->pmpro_membership_orders ORDER BY id DESC LIMIT 1'));

		//increment and check if it's available until we find one, this is probably goin
		$already_used = true;
		while($already_used != false) {
			$last_order_id++;
			$code = $prefix . $last_order_id;
			$already_used = $wpdb->get_var("SELECT code FROM $wpdb->pmpro_membership_orders WHERE code = '" . esc_sql($code) . "' LIMIT 1");
		}
	}

	return $code;
}
add_filter("pmpro_random_code", "my_pmpro_random_code", 10, 2);