Sure, there's a continuum of refactoring, from trivial to complete re-write.

Any time the data schema(s) change, you need to migrate. I'll bet that even when sticking with the same database flavor you'll need to migrate a handful of times over the first few months. Requirements change, blah, blah. After the first couple migrations, you refactor to make that less painful. Eventually it might get to the point that your persistence layer is fairly abstracted and you can change databases without ripping apart everything else. Doesn't happen with every project, but sometimes.