"What else is new here" is you can't have any downtime, and you have to have a strategy for having your code work against the data in all of those transitional

"What else is new here" is you can't have any downtime, and you have to have a strategy for having your code work against the data in all of those transitional states during that process. A lot of migrations would be pretty trivial if you could have even a couple hours of downtime, but in an expectation of 24/7 availability that is no longer acceptable in most situations.