Yes, you're free to change your plan at any time once you subscribe to Pro plan. You can easily upgrade, or cancel your plan for the next billing period through your account settings under Manage Subscription.
For cancellation, you will still have access to the service with Pro Pricing and Pro resources until the end of your billing period (No refund will be issued for the current billing period). After the end of your current billing period, you will be automatically downgraded to Hobby plan, which is our pay-as-you-go plan. All of your remaining funds will still be available to use in the Hobby plan.