Objectives: Migrating legacy on-premise electronic health record (EHR) systems in tertiary hospitals to modern cloud-native platforms presents technical and strategic challenges. We aimed to establish an optimized roadmap for transitioning legacy monolithic systems to a microservice architecture-based cloud-native EHR (MCEHR). Methods: We conducted a 3-month strategic assessment and case study based on the modernization requirements of a global healthcare provider. The methodology incorporated semi-structured interviews with key stakeholders, including clinical informatics officers and system architects, to identify critical pain points such as .NET 4.0 end-of-support risks and performance bottlenecks during peak clinical hours. Phased hybrid migration was adopted, analyzing over 20 TB of Oracle-based legacy data and evaluating the technical feasibility of transitioning to .NET 9 and RESTful APIs. To ensure clinical safety, a proof-of-concept (PoC) environment was developed to simulate high-concurrency clinical workloads, emphasizing system resilience and transaction integrity during intensive order-entry periods. Results: The transition to .NET 9 and MCEHR demonstrated 100% transaction integrity across 1,012 complex clinical test cases. Frontend and backend modernization showed high feasibility; however, migration of business logic embedded within legacy Oracle views represented a primary technical bottleneck, necessitating targeted decoupling. The PoC confirmed that RESTful API-based services maintained stable throughput under heavy concurrent loads, significantly reducing the risk of system-induced delays in clinical workflows. Conclusions Transitioning to an MCEHR architecture is complex but strategically essential. The proposed task force team roadmap outlines staged upgrades incorporating core technology modernization (.NET 9 and RESTful APIs), selective business component migration, and parallel DevOps adoption.