Changing or cancelling a plan
Change plan sits on each plan in Settings → Billing & Usage. What happens next depends entirely on whether you are moving up or down.
Upgrades apply immediately
Moving to a more expensive plan takes effect straight away. The new allowance is available at once, and you are invoiced for the difference immediately, prorated for the remainder of the period.
This is the behaviour you want when a meter is about to run out: the upgrade is the fix, and a fix that waited until the renewal date would not be one.
An upgrade also replaces a scheduled downgrade. If you had a downgrade waiting for the period end and then upgrade, the downgrade is dropped rather than applying later.
Downgrades wait for the period end
Moving to a cheaper plan is scheduled, not applied. You keep the plan you paid for until the end of the current period, and the cheaper one starts when it renews.
Nothing is refunded, because nothing is cut short. You have already paid for this period, and you keep what you paid for.
The screen confirms the change is scheduled rather than done.
A scheduled downgrade means your allowance drops on renewal day. If your usage sits above the smaller plan's limit, the day the downgrade lands is the day you start paying overage. Check the current period's usage against the new limit before scheduling, rather than after.
Undoing a scheduled change
A scheduled change can be undone before it applies, which puts you back to simply staying on your current plan. Once it has applied at the renewal, undoing is a fresh plan change rather than an undo.
Cancelling
Cancel this plan ends the subscription at the end of the current period, not immediately. Sending continues until then, and the screen says so.
Cancelling asks you to type cancel to confirm, because it is not a small change.
The two plans cancel separately. Cancelling marketing leaves transactional running, which is usually what you want: campaigns are seasonal in a way that password resets are not.
When a payment fails
If a payment does not go through, the plan says so and offers Resume in billing portal. Payment problems are fixed on Stripe's side, because that is where the card lives.
Sending is not cut off the moment a payment fails. Deal with it promptly anyway: the grace that keeps your mail flowing is not indefinite, and a card that expired is usually a two-minute fix.
Free workspaces
A workspace that has never had a paid plan has no subscription behind it, so there is nothing for a plan change or a cancellation to act on. Adding a plan is a checkout rather than a change, and the add-ons and cancellation controls only appear once a real subscription exists.