How payflow handle proration in upgrading subscription plans?

Question

I have scenario in pay flow recurring payment. suppose if a user subscribe for "Quick Start ($40)" then upgrade its subscription to "Advance Plan($80)", how much payflow will charge user on next billing cycle, $40 (Price difference) or $80 (Advance Plan Price). What if i want user to pay only price difference. Please help. i could not find any solution after day of googling.