Not necessarily. You can migrate old instances of a class to the new version of a class, but unlike a relational database, you don't have to. Unlike a relational db, an object db allows storage of many different versions of the same class to be stored. It's up to the application to decided if it needs a particular version to run.

You could for example add a new instance variable and change the way something is calculated and as long as the public interface of the class doesn't change you don't have to migrate the old instances; the new calculation could apply from this date forward.