Generally and for many things, augmenting the existing backend is the best way over a rewrite which would take more than 5 years for many systems and with that,

Generally and for many things, augmenting the existing backend is the best way over a rewrite which would take more than 5 years for many systems and with that, get shot down by accountants and management bureaucracy. As for the VM analogy, hmmmm from a data perspective, maybe. But it all boils down to how the old system stores data, and some will enable you to hold multiple records and for example a comments field. May have the last character closed of from input as it is a continuation marker that means there is another record and they are joined, as you say two records. Changes like that will entail changes to the backend, though the impact can be more manageable than alternatives. Then you can use replication or data warehousing and from that, lots of reports and other aspects that would hammer the backend become much more pleasant. Crux is the limitations, things like names, so entwined into a system that it does not lend itself to the two records solution. So you use another solution. You mention business rules and that is very much key and having somebody who knows the business logic is way more valuable to understanding the a system than knowing the language it is written in. So as an aside, if I had to employ somebody to work on an old legacy system, I'd take the person who knew or showed a grasp of the business logic and how that works over somebody who knew COBOL. As it is easier to teach COBOL than an entire business. COBOL has many books and resources, a business tends not to have such clear cut documentation.