Enable subscription changes for shipment-based plans

🔍 Problem Statement

Pelcro currently does not allow subscription plan changes when either the current or target plan includes shipments. When a subscriber wants to upgrade, downgrade, or switch between shipment-based plans, customer service must manually cancel the existing subscription and create a new one — increasing operational overhead and preventing self-service for subscribers.

💡 User Story

As a Pelcro admin, I want the platform and UI to support plan changes for shipment-based subscriptions, so that customer service teams and subscribers can switch plans without manual cancellation and re-creation.

🎯 Definition of Done (DoD)

A feature is done when:

✔️ Given a subscriber on a shipment-based plan, when a plan change is initiated via the Pelcro admin UI or by the subscriber directly, then the change is processed successfully — applying existing plan-change rules (billing alignment, proration) — without requiring customer service to manually cancel and recreate the subscription.