Lower the tier, cancel at the end of the period or switch to the spending cap right away; raising the tier, resuming a plan that is set to end or switching to top-ups from the card returns a confirmUrl for a person to confirm

Changes the plan without charging the card. These apply immediately, because they never raise what can be charged: lowering tier (it takes effect from the next period, nextTier shows it), cancelAtPeriodEnd: true (no further charge, the credit stays usable until periodEnd) and exhaustionMode: "cap" (also turns auto top-up off). Raising tier, cancelAtPeriodEnd: false on a plan that is set to end (resuming it) and exhaustionMode: "topup" do NOT apply: the call returns 409 subscription_confirmation_required and nothing in the request changes. The error body carries confirmUrl, a link to the portal page where a person signed in to the account confirms the change; the link is valid for 24 hours and works once. Send changes that need confirmation one per request. A request that changes nothing (resuming a plan that is not set to end, topup while top-ups already run) succeeds without a confirmation.