Here tips from me :

/ back up before transitioning into new system

/ when the system

Build by COBOL can stand for centuries since the 1960, it’s proven the system are resilient, you may want to keep it and instead add a little by little new code in … so basically instead of moving the old system COBOL to the new-system and have risk to impact million people , it’s better man injected new system into the old system .. per bit . What you get is upgraded COBOL version .

/ sometimes new language systems is not as good