Switch an existing subscription to another plan (principal-scoped)

The caller's own account. Modifies the existing subscription in place — up or down — with create_prorations in both directions, settled on the next invoice rather than charged today. A downgrade applies immediately, including its smaller quota ceiling.

enterprise and free are not switch targets: the first is quoted and hand-provisioned, the second is what cancelling leads to.

The response reports what STRIPE accepted. plan on the billing summary is written by the customer.subscription.* webhook a moment later, so a summary fetched in the same breath may still report the old plan — render the value returned here and re-fetch the summary shortly after, rather than treating the lag as a failure.