You can change your subscription type at any time. On your account page under in the "Subscription" section under "My account", you can see which subscription you have at the moment and you can click "Change subscription" to see the other options and to change what you have.
If you downgrade your subscription, the change will take effect from your next payment date. You keep your current subscription type until the current payment period is over.
If you upgrade your subscription from Basic to Standard or from Standard to Premium, you pay the price difference to the higher price calculated by how many days that are left in your current period and you get access to more hours instantly. From the next payment date we will automatically charge the new price if you don't choose to cancel or downgrade again until then. If you have Premium and use up your hours, you can buy extra hours on your account page to keep listening and reading until the end of your current period.