Yes, you can upgrade, downgrade, or change the number of admin users and operators at any time. Once you do, your billing details are recalculated automatically. If you are downgrading, you will get credit for the part of the subscription fee and unused add-ons from your Pro plan.
If you choose to be billed monthly, you can cancel any time under the Subscription section in your Katana account settings. If you connected to Katana from the Shopify App Store, your billing is handled by Shopify. To cancel, simply uninstall the Katana app from your Shopify account.