Migrating relational data from Oracle RDBMS to Neo4j may be necessary when an existing application reaches its performance limits, requiring a new application or significant development. This can occur due to slow application performance, complex queries, or the need for real-time joining of varying sets and depths of data. In such cases, migrating all data to Neo4j can greatly improve query times and simplify connected data queries. Companies like Schleich GmbH and Billes have successfully migrated their traditional systems to Neo4j, achieving improved flexibility, performance, and operational efficiency. The process involves editing application code, rewriting queries, and leveraging Neo4j's flexible data model for seamless system integration.