Meta's data center networks have relied on BGP for routing and interconnectivity due to its scalability and simplicity. However, as our network evolves, BGP's limitations in supporting complex network migrations have become apparent. These migrations require customized routing at each intermediate step, considering topological properties. In this paper, we highlight the unique challenges of production migration and illustrate how native BGP falls short, unable to encode both sequential and spatial conditions. To address this challenge, we introduce a novel Route Planning Abstraction (RPA) that augments the BGP protocol to support migration. It enables centralized route planning while maintaining distributed enforcement. We demonstrate the power of this abstraction by developing Centralium, a hybrid route-planning framework with a centralized controller, and over ten use cases to support various migrations in production. Our production experience with Centralium, deployed alongside BGP in large-scale data centers, has shown substantial reduction in the time and risk of network migration operations.