Redirect the Paid Memberships Pro confirmation to a page for a specific upsell unless the user already has that level.

pmpro_confirmation_url_to_upsell.php

<?php
/*
	Redirect to a URL promoting an upsell unless the user already has that level.
	
	Add this code into a custom plugin.
*/
function pmpro_confirmation_url_to_upsell($rurl, $user_id, $pmpro_level) {
	$upsell_level_id = 2;							//change this
	$upsell_url = 'https://example.com/upsell-page/';			//change this

	if(!pmpro_hasMembershipLevel($upsell_level_id, $user_id))
		$rurl = $upsell_url;

	return $rurl;
}
add_filter('pmpro_confirmation_url', 'pmpro_confirmation_url_to_upsell', 10, 3);