plan_migration_unavailable
HTTP 422, type invalid_request_error.
Changing a subscription's plan is not available yet.
Why it happened
A plan change needs proration, which is not available yet.
How to fix it
Cancel the subscription NOW, with POST /subscriptions/{id}/cancel and cancel_at_period_end: false, then create a new one on the new plan. The default cancellation waits for the end of the period, so the old subscription would keep billing while the new one also bills, charging the customer twice. Or contact support.
Retrying: do not retry the same request. It will not succeed.