completely understandable. good example you mentioned there. imagining how software can put people's life into danger is really crazy. little bit softer could be in banking system  that might corrupt transaction and securities involved, but some might get heart attach. I've seen people getting excited about new tech but the risk is the biggest challenge.

so going back to the original question, our friend should really clarifies the type of migration  and provide milestones for each step not to impact the serviceability of the running COBOL system. eventually he might get into a full migration. It might also be partial COBOL and partial new tech (whatever that new tech is). there is no straight guideline for that other than prototype, test, and be careful.