I would strongly disagree with that, do not go into a migration with the expectation that you'll impact people. If you do that, you'll take shortcuts, you'll st

I would strongly disagree with that, do not go into a migration with the expectation that you'll impact people. If you do that, you'll take shortcuts, you'll start thinking in the wrong ways. Suddenly you'll start saying to yourself that the migrated customers should be able to live with X or Y or that your colleagues have to accept that they have to do these various steps because hey, we are doing a migration after all. Instead it has to retain the exact same behavior at all times. It should cause zero pain whatsoever, if it causes pain you failed at your migration task. Secondly I agree with the other poster that it has to be incremental, otherwise you might as well accept a monumental amount of bugs from the start. My third point is that you should automate as much as possible and write code to do the migration in a repeatable way, first on testdata, and then keep expanding the type of testdata until it encompasses all the possible data that customers can have. Then you run that migration on the press of a button and it should work perfectly every single time you do it.