Yes, you can change your plan. You could either upgrade or downgrade your subscriptions without any restrictions or limitations.
With reference to Monthly plans:
If you want to upgrade your subscription, you will be paying a specific amount for the entire month. If you want to downgrade your subscription, your subscription for the next month will be reflected to the current plan.
With reference to Annual plans:
If you want to upgrade your subscription, you will have to pay a separate annual and due amount in a new bill. If you wish to downgrade, you will have to wait till your plan is completed.
To conduct the changes in your subscription, you may access via Settings >> Billing >> Plan