Proration

A subscriber on a $9.99 monthly plan upgrades to a $19.99 annual plan on day 10 of a 30-day cycle. Twenty days of the monthly plan go unused, worth about $6.66, and proration decides what happens to that $6.66. It might come off the annual charge, or convert into extra time on the annual term, or sit untouched while the upgrade waits for the current period to end.

The arithmetic is the same everywhere. The resolution is not. Stores differ on whether a plan change takes effect immediately or at the next renewal, on whether unused value returns as a credit or as time, and on how much of that behavior a publisher gets to choose per transaction. Apple, Google, Roku, and web billing platforms each answer differently, so one upgrade button can produce different receipts depending on the surface it was tapped on. Downgrades tend to be more constrained than upgrades, and changes that cross a currency or a store more constrained still.

Proration matters at the experience layer because the upgrade screen makes a promise about money. Tell a subscriber “upgrade for $19.99” when the store is about to charge $13.33 today, or nothing today and $19.99 in twenty days, and the transaction has been misdescribed. The store receipt is where they find out. Getting that copy right per platform is a design problem before it is a billing problem, which is why plan-change screens belong with the rest of the pages and flows on the platform rather than hard-coded once per store. What the subscriber holds after the change settles is an entitlement.