You can always upgrade to a higher plan. If you need more training seats than anticipated, simply upgrade. At the moment of upgrading you can change your payment period from monthly to annually and 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 and training seats for the remaining period of the previous plan will be settled with the upgraded plan.
You are on a Lite plan with an annual payment period. After month 6 you decide to upgrade to a Medium plan. In the first 6 months you have used 180 of the 200 training seats available in the Lite plan.
At the moment of upgrading you have used 50% of the subscription period (6 months out of 12 months). Therefore you only need to pay 50% off the price for a Lite plan.
Since you paid the whole plan at once (annual payment), you get a 50% refund.
For this shorter subscription period you also have 50% less training seats included. The additional training seats that were used outside of the previous plan will be deducted from the new upgraded plan.