Comment by mjevans

Yes, that's why the new system has to be a complete replacement. Part of it's spec COULD be to provide backwards interfaces too, in case things can't all be cutover at once, but that would increase the project scope and also tie things down to the old system too.

Part of a full replacement system would be the option to use a _different_ set of rules, which better reflect current desires and are, hopefully, easier to implement.

Yes the old data would need to be _transcribed_ during it's restoration to the new system, and human bureaucratic layers can likely handle issues. Heck, they could do a deferred implementation of the new system where one long weekend the new system's brought up, and any of the issues that are noticed as kinks worked out. When there aren't any _noticed_ kinks in those tests have the results sent out to the stakeholders and solicit feedback on if there are any inaccuracies. Which might take a year or two of renewals and updates and the annual business as they see if the new notices are correct or not.