Legacy .NET systems usually aren't "old code" so much as years of production rules, edge cases, integrations, and compliance work that the business still depends on. The safest modernization plan is often not a rewrite — it's a staged path that keeps the working system alive while you peel off risk one piece at a time. This is a developer-to-architect walkthrough of how to choose between infrastructure upgrades, APIs, strangler-fig replacement, and direct application bridging when a full rewrite is too risky. Originally published on the JNBridge blog . The Legacy .NET Modernization Challenge Enterprise .NET applications often contain 10-20 years of accumulated business logic , regulatory compliance features, and integration points that represent millions of dollars in development investment.…