Large-scale network modernization can increase capacity, resilience, and readiness for future services, but migrating live services introduces significant operational risk. This study examines the consolidation of two independent IP/MPLS production networks in the Airtel–Tigo Ghana national network, which served approximately 10 million subscribers. The modernization increased key backbone capacity from approximately 30 Gb/s to 100 Gb/s while live services were progressively transferred to the consolidated architecture. The study formalizes a Safe Migration Design Framework based on four operational principles: a migration should be easy to execute, easy to troubleshoot, easy to roll back, and easy to complete. The migration achieved minimal service risk and near-zero disruption while maintaining critical services. The lessons provide a practical framework that network engineers can apply to similar largescale brownfield network migrations.