Tricky bit with ancient codebases is that their only requirements, generally speaking, tend to be that they should keep working exactly like they have been since 1983, except for the bit that needs changing of course, that needs to change in a way that implements the change, but doesn't have any unintended side-effects in a system that is a fractal of undocumented interdependencies (systems that have been patched for a few decades under those types of constraints tend to become especially gnarly that way).