Downgrade your plan
Downgrading starts the same way as upgrading: the “Billing” page, the “Subscription” card, then “Change plan”.
When it takes effect
A downgrade currently applies immediately, with the same proration as an upgrade. The unused part of your current plan is credited and the new plan is charged for the rest of the period.
The confirmation says so explicitly before you commit, and it warns you of the consequence that matters: the lower plan’s allowances, limits and overage rates apply right away. You do not keep your old plan’s allowances for the rest of the period.
That is the behaviour to plan around. If you are part way through a busy period on a higher plan, a downgrade will change what you can do that same day.
A scheduled change that already exists
If your account has one, the “Subscription” card shows the plan it moves to and the date it takes effect, and tells you that you keep your current allowances and pricing until then and that nothing is charged or refunded now.
“Cancel scheduled change” calls it off. You stay on your current plan and nothing is charged.
Moving to Free
Going from a paid plan to Free is a cancellation rather than a plan change, and it is handled separately. See Stay on Free.