How do upgrades work?
Money paid on your current tier will be used to discount the upgrade cost - i.e. proration. If you've paid $15 on starter, and upgrade to pro ($50) immediately, $15 will contribute to the upgrade - you'll pay $35. If you've paid $15 on starter, and upgrade to pro 2/3rd through the month , $5 will contribute to the upgrade - you'll pay $45.