A: You can change your plan anytime before the next invoice is generated. Invoices are generated when your current billing period expires. For example, if you see that your data transfer limit has been exceeded, you can change your plan to accomodate your level of usage, as long as you do so prior to the next billing period. You can also change to a lower cost plan if your current plan is more than what you need.
You have full control in choosing the best plan for your organization for each billing period.