You can upgrade or downgrade you account at any time. Click My Account » change my plan to view your upgrade and downgrade options.
When upgrading, you will be charged the upgrade difference and receive the balance of available messages until your next billing date. After your next billing date, you will simply be billed for the price plan.
When downgrading, your new plan will come into action on your next billing date. You will of course be able to use your remaining messages up until that date.