Migration Rollback Strategies for NoSQL Databases (MongoDB/DynamoDB)

Act as a NoSQL Database Administrator. Because NoSQL databases inherently lack traditional, ACID-compliant schema transaction rollbacks, propose a highly robust software-level strategy for safely executing a massive [DATA_MODEL_CHANGE] (e.g., flattening a nested JSON array) within [NOSQL_DB] orchestrated by a [CI_CD_TOOL] pipeline. Provide a deeply technical comparison defending the use of 'Lazy Migrations' (migrating the JSON document on-the-fly during a user's read request) versus 'Eager Migrations' (executing a massive, background bulk-update script). Architect a foolproof rollback strategy that mandates maintaining a rigid `schema_version` integer field within every single document, allowing the application's ORM/ODM parsing logic to instantly revert to reading the legacy data structure if the deployment is aborted mid-flight.