This is actually the case, old and new schemas are available and working until you complete the migration, and you can run this step whenever you want.
The aim is not to deal with conditional logic in the app dealing with both schemas, but having an old version of the app linked to the old schema and the new one using the other.
so if I want to sunset my api version X in 1 year for whatever reason and I am able to support an old schema X, which api X maps to, for that time period without any hassle (and not only during migration), this would be a much bigger feature / USP for me then everything else mentioned. I am really curious to look deeper into this :-)
The aim is not to deal with conditional logic in the app dealing with both schemas, but having an old version of the app linked to the old schema and the new one using the other.