What does it take to port code from one language to another without users noticing it? During the multi-year process of porting more than 100k lines of code from Classic ASP to C# we learned a lot. How do you plan and approach project at this scale? How do you test such a large and complicated codebase? What are rollout and rollback strategies? The project succeed, but there were some costs and lessons learned. We know how to do it again and we learned what not to do.