The scariest sentence in software is "we will switch over on the weekend." A single big-bang migration bets the whole business on one Saturday going perfectly.

It rarely does. Data that looked clean turns out to have twenty years of exceptions, and by Monday morning nobody can tell what moved and what did not.

We migrate in stages instead. The old system and the new one run side by side, one module at a time moves across, and each move is reversible. Reporting reads from both until the numbers agree.

It takes longer on paper and far less in practice, because nothing has to be perfect on a single night. The business keeps running the whole way through.