All changes to Proton VPN and Proton Mail plans are immediate and irreversible. This means that if you've changed your plan, you won't be able to go back to the previous state. This is done for the sake of security. If you have an unused amount when you make modifications to your subscription, it will be added to your account as credits, which you can use at any time to make payments or subsequent upgrades to your plan. Details are available here.