You may upgrade at any time in case your needs change.
To view your upgrade options and upgrade your account, you will need to log into the client area. Once you are in the client area, click on MY PRODUCTS and click on MANAGE for the account you wish to upgrade. On this page, you will see a button for UPGRADES.
Unfortunately, refunds cannot be given when you downgrade your account, but you may do so effective at the beginning of the next billing cycle.