I've rewritten an old application that was a rewrite of a mainframe (prob COBOL I guess). There were quirks that we still had to carry over. Luckily there were

I've rewritten an old application that was a rewrite of a mainframe (prob COBOL I guess). There were quirks that we still had to carry over. Luckily there were some old hands that can sometimes tell you why and we didn't always have to look into the CVS history and find bug-1376892. For testing you'd run a few weeks of data through and make sure they match outputs exactly. In prod we'd run them in parallel a bit for each report before decommissioning it. There'd always be something like..at the end of 3rd quarter if the last day is a Friday, some other junk happened and now your data looks different. After a few we got a hang of which things were likely to break and was able to have certain parts driven by configuration and many could be fixed via an updating a config if a bug was found.