post
https://sandbox.api.tansoflow.com/api/v1/client/subscriptions//plan-change
Upgrades or downgrades a customer's subscription to a different plan.
Upgrade behavior: Takes effect immediately. A new invoice is generated for the upgraded plan.
Downgrade behavior: Scheduled for the end of the current billing period. The customer keeps their current plan until the period ends, then transitions to the new plan automatically.