There is no downgrade option. So if you require another site license for a short term use, its better to buy another license. Keep in mind that refunds are allowed only within 30 days and for once. Basically you can’t keep buying and refunding every 30 days.
You will receive notification emails about your license expiration along with the cost of renewal. Currently renewals are happening at a 40% discounted rate if you renew before expiration and 20% after expiration.