1. Database Setup

• 1.1 Create subscription_downgrade_schedule table migration
• 1.2 Add stripe_subscription_item_id column to subscriptions table
• 1.3 Create plan_priorities configuration table
• 1.4 Seed plan priority data for AI/VC/Care plans

2. Backend - Core Infrastructure

• 2.1 Create plan priority service for upgrade/downgrade detection
• 2.2 Implement plan comparison logic (same/upgrade/downgrade/new)
• 2.3 Add Stripe subscription update utility functions
• 2.4 Create subscription state sync service

3. Backend - API Endpoints

• 3.1 Implement GET /api/subscription/check-upgrade endpoint
• 3.2 Implement POST /api/subscription/upgrade endpoint with Stripe integration
• 3.3 Implement POST /api/subscription/schedule-downgrade endpoint
• 3.4 Implement DELETE /api/subscription/schedule-downgrade endpoint
• 3.5 Add API request validation and error handling
• 3.6 Add comprehensive logging for upgrade/downgrade operations

4. Backend - Webhook Handlers

• 4.1 Implement customer.subscription.updated webhook handler
• 4.2 Implement customer.subscription.deleted webhook with downgrade execution
• 4.3 Implement invoice.payment_failed webhook handler
• 4.4 Add webhook idempotency protection
• 4.5 Create webhook event logging and monitoring

5. Backend - Device Binding

• 5.1 Implement device binding release logic for downgrades
• 5.2 Create device rebind notification service
• 5.3 Add device count validation on downgrade

6. Frontend - UI Components

• 6.1 Create upgrade confirmation modal component
• 6.2 Create downgrade confirmation modal component
• 6.3 Implement duplicate purchase blocking UI
• 6.4 Add button debounce (1.5s) to all Paywall CTA buttons
• 6.5 Create downgrade scheduled status display

7. Frontend - Integration

• 7.1 Integrate upgrade check API on Paywall CTA click
• 7.2 Connect upgrade confirmation to upgrade API
• 7.3 Connect downgrade confirmation to schedule-downgrade API
• 7.4 Implement downgrade cancellation UI flow
• 7.5 Add Apple Pay / Google Pay support to Paywall

8. Testing - Backend

• 8.1 Write unit tests for plan priority service
• 8.2 Write unit tests for upgrade/downgrade detection logic
• 8.3 Write integration tests for upgrade API endpoint
• 8.4 Write integration tests for downgrade scheduling API
• 8.5 Write webhook handler tests with mock Stripe events
• 8.6 Test Stripe subscription update with proration
• 8.7 Test device binding release on downgrade

9. Testing - Frontend

• 9.1 Write component tests for upgrade confirmation modal
• 9.2 Write component tests for downgrade confirmation modal
• 9.3 Test button debounce functionality
• 9.4 Test duplicate purchase blocking flow
• 9.5 Test Apple Pay / Google Pay integration

10. Deployment Preparation

• 10.1 Create feature flag for upgrade/downgrade functionality
• 10.2 Set up monitoring dashboards for subscription changes
• 10.3 Configure Stripe webhook endpoint in production
• 10.4 Create rollback runbook
• 10.5 Document API endpoints for frontend team

11. Deployment & Monitoring

• 11.1 Deploy database migrations to staging
• 11.2 Deploy backend API to staging
• 11.3 Deploy frontend changes to staging
• 11.4 Execute end-to-end testing on staging
• 11.5 Enable feature flag for 10% users (AI Paywall)
• 11.6 Monitor error rates and conversion metrics
• 11.7 Gradually roll out to 100% users across all Paywalls
• 11.8 Set up alerts for webhook failures