Comment by Analemma_
This generally isn’t true in the case of government systems. For the most part they are performing tasks that are required by law, and it is not acceptable to stop some of them, even temporarily. If you’re lucky you can run the old and new systems side-by-side while the 100% feature migration occurs, but that isn’t always feasible.
Ya it's funny looking at all these 'business' programmers that if the application doesn't work can just loose the customer/money to another competitor. In regulated stuff you have to serve everyone. Much worse if your systems don't work there are potential consequences where people die and or there are riots in the street.