You can, at any time, upgrade to a Medium, Pro or Enterprise plan. If you need more active players than anticipated, simply upgrade. At the moment of upgrading you can also change your payment period from monthly to annually or the other way around. All upgrades will be effective immediately.
When you upgrade to a higher plan during an active subscription period, a new full-year subscription period starts. Costs for the remaining period of the previous plan will be settled with the upgraded plan.