Anyone who has worked on a large migration eventually lands on a pattern that goes something like this:
1. Double-write to the old system and the new system. Nothing uses the new system;
2. Verify the output in the new system vs the old system with appropriate scripts. If there are issues, which there will be for awhile, go back to (1);
3. Start reading from the new system with a small group of users and then an increasingly large group. Still use the old system as the source of truth. Log whenever the output differs. Keep making changes until it always matches;
4. Once you're at 100% rollout you can start decomissioning the old system.
This approach is incremental, verifiable and reversible. You need all of these things. If you engage in a massive rewrite in a silo for a year or two you're going to have a bad time. If you have no way of verifying your new system's output, you're going to have a bad time. In fact, people are going to die, as is the case here.
If you're going to accuse someone of a criminal act, a system just saying it happened should NEVER be sufficient. It should be able to show its work. The person or people who are ultimately responsible for turning a fraud detection into a criminal complaint should themselves be criminally liable if they make a false complaint.
We had a famous example of this with Hertz mistakenly reporting cars stolen, something they ultimately had to pay for in a lawsuit [1] but that's woefully insufficient. It is expensive, stressful and time-consuming to have to criminally defend yourself against a felony charge. People will often be forced to take a plea because absolutely everything is stacked in the prosecution's favor despite the theoretical presumption of innocence.
As such, an erroneous or false criminal complaint by a company should itself be a criminal charge.
In Hertz's case, a human should eyeball the alleged theft and look for records like "do we have the car?", "do we know where it is?" and "is there a record of them checking it in?"
In the UK post office scandal, a detection of fraud from accounting records should be verified by comparison to the existing system in a transition period AND, moreso in the beginning, double checking results with forensic accountants (actual humans) before any criminal complaint is filed.
[1]: https://www.npr.org/2022/12/06/1140998674/hertz-false-accusa...