Core API: Change Plans for Active Subscriptions

🔍 Problem Statement
As a developer, I experience the plan change feature (v1.84) only being available in the dashboard UI, which prevents bulk plan changes for 100+ subscriptions and requires error-prone manual processing.

💡 User Story
As a developer, I want to change the plan on an active subscription via the Core API, so that I can bulk-update subscriptions programmatically.

🎯 Definition of Done
- Given an active subscription, when PATCH /v2/subscriptions/{id}/plan is called with a new plan ID, then the plan is changed and a confirmation is returned.
- This change impacts the API (subscription plan change endpoint). Must be documented and tested.
- Limitations: same business rules apply as the dashboard (proration, eligibility); no bulk endpoint in v1.

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What would you like to be able to do? (One idea per form): Be able to leverage the CORE API to use the feature implemented in Release 1.84 -- Feature: Ability to Change Plans for Active Subscriptions How would that help you?: We often times have a need to change the plans for active subscriptions in bulk for 100+ subscriptions. Having this ability in the CORE API would allow us to do this in bulk versus one at a time. How important is this to you?: Critical Email address: mpatel@crain.com ——————————————— This task was submitted through Suggest an idea - Product Roadmap https://form.asana.com/?k=qsv6hMm2wL8SXD_R-h-N9Q&d=742417627358732