What we don't do is look at the old code, document how it works, and then reproduce that. Prior to this legacy system being built the entire company worked with paper processes and documentation, so it was a paradigm shift for how the business worked. That system is slow to update, so how they work is heavily influenced by the business' thinking 40 years ago. Our replatforming project is seen as essential for the business' continual survival, so we're allowed to question processes, simplify where we can, and work as equals with the business in defining new processes. There are definitely hold-outs and resistance from some quarters, but once you launch some successes people start converting and accepting the process.
it's also probably very different whether the system has been continuously adjusted and fixed for 40 years, or if it's just stayed there.
For example, what was once a file based batch process meant that other processes had to wait and split their processes accordingly. Replace that with an event based system and everything can run contiously and a lot of the restrictions disappear.