I may be wrong, but to me it seems like they will replace the current implementation with something fundamentally incompatible. My impression is that dependencies like RMI will go away, too. But that would be a huge breaking change.
Sure it's going to be incompatible and break old code. The title, however, makes it seem like Oracle is getting rid of serialization when it's just replacing it with a safer method.
Just for the sake of arguing semantics: "Just replacing" is incorrect in my view. The new implementation is designed specifically to not be one. It is a different mechanism mit a different interface and a different design goal.