Upgrading & downgrading
Upgrades apply now with proration; downgrades wait for your renewal date.
User guide
Upgrades apply now with proration; downgrades wait for your renewal date.
The direction of a plan change decides its timing: up is immediate, down waits for the period you have already paid for to end.
Who can do this: the Administrator, from Billing → Change plan.
Choosing a higher plan shows exactly what you will pay before anything happens:

Press Confirm and pay and the upgrade applies immediately — new features unlock on the spot. If you have no card on file yet (upgrading from Free), you go through the secure Stripe checkout first, exactly as at signup. If your bank asks for extra confirmation, the screen hands you Stripe's payment page and the upgrade lands when the payment confirms.
Choosing a lower paid plan is scheduled, not immediate: "Your plan changes to {plan} on {date}. Nothing changes until then." You keep everything your current plan includes for the period you already paid for.

Until the date arrives you can change your mind with Cancel downgrade. Two consequences to read before scheduling:
That is cancellation, and it has its own page: Cancelling & moving to Free. While a move to Free is booked, other plan changes are refused until you cancel the booking.
Comparing plans · Seats & add-ons · Payment & the customer portal