Credit one level's cost to another level at checkout with PMPro.

pmpro_credit_on_upgrade.php

/*
	Upgrade Pricing. Make sure to change "$level->id == 15" to check for your own level ID.
*/
function my_pmpro_checkout_level($level)
{
	$user_level = pmpro_getMembershipLevelForUser();
	
	//if($level->id == 15 && pmpro_hasMembershipLevel(6))	//to check for a specific level combo
	if(!empty($user_level->initial_payment))
	{
		max(0, $level->initial_payment = $level->initial_payment - $user_level->initial_payment);
	}
	
	return $level;
}
add_filter("pmpro_checkout_level", "my_pmpro_checkout_level");