Yes, you can change the type of subscription after the current subscription expires.
If you are on a monthly subscription, you will first need to cancel the current subscription to the course in the subscription settings on the website. Once the current subscription runs out, go to the Subscriptions page and choose a different type of subscription for payment.
If you are on an annual subscription, you will need to wait until it runs out, and only then pay for a different type of an annual subscription.
There is no option to upgrade the subscription by paying for the cost difference.