When I change service package, add or delete units, how's the account subscription expiration recalculated?
Service credit (or subscription days) are shared among all units in your account. When you change from one package to another in "Account Settings", or add/delete units from "Manage Units" page, the subscription expiration date will be recalculated based on the remaining credit, new package price and the total number of units in the account.
For example, you have 1 unit under your account with 2 months service subscription, when you add a second unit to the account, the 2 months service credit will be distributed to 2 units and the subscription will be automatically adjusted to 1 month. Your total service credit amount will be unaltered before and after adding/removing units in your account. In this case, it was 2 months service for 1 unit, now it's 1 month service for 2 units.
To change your package online anytime: after login, click "Account", then click "Account Settings" and change package there. To add/delete unit, go to "Manage Units" page.