I have been part of couple of migration projects from COBOL (and accompanying mainframe tech) to newer tech. COBOL is a very simple language. What makes porting

I have been part of couple of migration projects from COBOL (and accompanying mainframe tech) to newer tech. COBOL is a very simple language. What makes porting next to impossible is the functional knowledge of the application, how it intracts with uostream/downstream systems, all the special case handling and how business users expect the new system to behave (hint: same as the existing system). Biggest challenge to porting is that you can't port an entire system at once (takes years), so when you starting migrating part by part, it has to interact with its neighbouring systems exactly how the old system worked. And that is what is most difficult part of porting IMHO..