There might be legal / compliance reasons. It can be incredibly difficult to replace a validated system that is known (or already accepted even if it's technically incorrect) to implement lawmaker dictated behavior.
Otherwise, I think a new approach might be to ignore the specifics of the old system, implement a new system, and a separate translation layer that can run on an export of the old system (or the old system brought back online, but read only after the overnight maintenance) and completely cut over during an otherwise holiday weekend.
Otherwise, I think a new approach might be to ignore the specifics of the old system, implement a new system, and a separate translation layer that can run on an export of the old system (or the old system brought back online, but read only after the overnight maintenance) and completely cut over during an otherwise holiday weekend.