Switching plans or billing cycle
If you've hit the limit of your plan or want to switch between yearly/monthly billing, head over to Account > Change Plan (requires Billing permission). That will let you change to a different plan and change your billing cycle.
When you upgrade to a more expensive plan or from a monthly to a yearly billing cycle, your current plan is prorated for the remaining number of days. When choosing your plan, the exact amount that will be prorated is shown to you before confirming the plan change. When you confirm the change, an invoice is immediately generated and charged to the payment method on file. The next renewal date for your account is set to today plus a month or year.
When downgrading to a cheaper plan, and/or from yearly to monthly, your current plan is converted into time on the new plan. The exact calculation is shown before you confirm the plan change, and in most cases no payment will be due. The plan change is processed immediately and your next renewal date is set accordingly.