You can prepare your server app for migration using the Atlassian app cloud migration library.
This guide will help you:
Enable app migration in your server app.
Upload your app-related data to Atlassian's secure cloud storage.
Get notified by the app migration platform when core data migration is complete and app data migration starts.
This lets you access/read mappings related to core product data (you can access the same mappings by making a request from your cloud site).