Upgrades are billed immediately and on a prorated basis. When you upgrade, you will only pay the difference between your old plan and your new plan if you’re upgrading in the middle of a billing cycle. Your billing date will stay the same as your original subscription date. If you cancel your plan before upgrading, a billing event will be triggered, and you’ll initiate a new billing cycle.
Downgrades do not trigger billing events unless you cancel your plan before subscribing to a lower-tier plan. Additionally, downgrades are not prorated, which means you won’t retain credit towards a lower-tier plan if you’ve paid for a higher-tier plan and downgraded before the end of your monthly/yearly billing cycle. It may thus, be wise to downgrade towards the end of your billing cycle, should you choose to downgrade at all. Finally, downgrading does not reset your billing cycle, so you can expect to receive your bill at the same time as your initial higher-tier plan, provided you haven’t cancelled your current plan before downgrading, as mentioned above.
To avoid unnecessary charges, avoid cancelling your current plan before upgrading/downgrading. Instead, simply change to your preferred plan while your current plan is active.
You can read more about upgrading or downgrading your subscription.