Proration/Adjustment/Partial Credit for Plan Upgrades

Ontraport needs a way for customers to be able to "upgrade plans". For example:

A Customer purchases an annual "Plus" product for $100/yr on January 1st. 6 months into the plan, on June 1, customer upgrades to "Pro" product which is normally $200, but because they are already 6 months into another annual plan they should be credited $50 making the upgrade total $150.

I've used another CRM that has transactions and subscriptions like OP(called TimelyBill) and how they did it was having an additional Object called "Adjustments" that applies a credit to the next transaction or open order payment. Their system also has an order form that handles that adjustment on the page so the customer knows what they're paying today for an upgrade.