Yes, you can change the modules you're subscribed to on your own, at any time, in the Subscription Management page found in the Settings view.
If you subscribe to a paid module, changes will take effect immediately. If you're already subscribed to a paid module you will be charged the prorated amount to sync up with your existing billing date.
If you stop subscribing to a module, changes will not take effect until the beginning of the next renewal date. For monthly plans, that means the next month's bill date. For annual plans, that means on the annual renewal date.