One upside to writing an intermediate version that supports both the old and new representation: your data migrations are not an urgent race against the clock,

One upside to writing an intermediate version that supports both the old and new representation: your data migrations are not an urgent race against the clock, or mission critical to the release. You can decouple migrations from the release, and rest easy knowing your app is working without praying that a massive migration happens successfully and promptly. Also since your UI or admin Interface likely lets you inspect your data in either representation, you can more naturally verify up/down migrations work in staging.