There is no programmatic way of doing this. You need to have guys with domain expertise to help you through. Obviously, the effort to fix/migrate will always be

There is no programmatic way of doing this. You need to have guys with domain expertise to help you through. Obviously, the effort to fix/migrate will always be proportional to the time it took to create such a mess. This is 100MM lines, it never will be easy. I would take some time to create the tooling to do this. Say, create a tool to add some bytecode to generate a pretty callgraph. Then, I'd run the use cases or functionalities individually, and save the callgraph somewhere. But in the end, you will always need domain knowledge expertise to guide you through the logic of it.