Yes, you can cancel, upgrade or downgrade your subscription any time.
If you cancel, your subscription will be cancelled at the end of your current billing period, your remaining credits will be preserved for 6 months. You’ll be able to use them if you subscribe again.
Upgrading your plan takes effect immediately: after confirmation, your payment method is charged, you immediately get the credits from the larger plan, your renewal date gets updated to the current date. For instance when upgrading from 1000 credits / month to 5000 credits / month, you immediately get 5000 credits and your plan will renew in exactly one month (unless you cancel).
If you're downgrading your plan you will not billed immediately. Instead, your current plan stays active until the end of the current billing period, and only then the new plan takes effect. For example, when you downgrade your 10000 credits per month plan to the 5000 credits per month on June 10, and your next billing date is July 5, then you will be billed for it on July 5. This is also when the new roll over limit applies, so make sure you use the remaining credits until then.