Can I change my premium plan?

Yes, you can change your subscription. If you go into the Your Plan section on the Account page, you will see a button marked Change Plan:

If you click on this, you will be able to upgrade your plan from the one you are currently on to a subscription plan that has a longer duration, as seen here:

If you update your subscription you will be issued a prorated refund for the current plan’s duration and have it applied as a discount to the upgrade.