Enable subscription changes for shipment-based plans

🔍 Problem Statement
Currently, Pelcro does not allow subscription changes between plans that include shipments. This limitation forces manual intervention and increases workload for customer service teams, while also preventing subscribers from managing their own plans.

💡 User Story
As a Pelcro admin, I want the platform and website to support plan changes for shipment-based subscriptions so that customer service doesn’t need to manually cancel and recreate subscriptions.

🎯 Definition of Done (DoD)
A feature is done when:

✔️ The Core API and UI allow subscription changes between shipment-based plans.
✔️ All validations are consistent with existing plan change rules (e.g., billing cycle alignment, proration if applicable).
✔️ End users can perform plan changes directly through the UI without requiring customer service intervention.