When you change your plan the unused portion is calculated (prorata) and credited to the cost of the new plan which starts immediately. If the prorata amount is greater than the cost of the new plan (in the case of a downgrade) then the remainder is credited to your account and used on future invoices.
If the prorata amount is less then the remainder of the invoice is charged to the credit card on file for your account.
Note: Billing will be moved to purely usage based so there will no longer be separate plans