You might want to promote an object because new information has become available about it, and you need new fields to store that information. You might want to promote an object to add temporary computed fields, baggage you don’t normally carry around.

To promote an object in Java, you need copy Dalmatian copy constructor that takes a Dog parameter. Internally it must copy field by field. It might be implemented very slowly with reflection. It might be possible to implement it with JNI that was more aware than usual of the platform specific implementation. If promoting were built into Java, it could be implemented with a block move.