When do I have to change the serialVersionUID?

The value of the serialVersionUID field should ideally be changed when incompatible changes are made to the structure of the class. The complete list of incompatible changes is present in the Java Object Serialization Specification.

To expand further, incompatible changes to a class will prevent the deserialization mechanism from creating an instance of the object, because there is information in the stream that does not map to the current class definition.

The frequently-repeated mantra about changing the serialVersionUID every time you change the class is complete and utter nonsense. See this Sun article which they republished on their site and which was migrated to the Oracle Technology Network after the acquisition.

You should change the serialVersionUID only when you deliberately want to break compatibility with all existing serializations, or when your changes to the class are so radical that you have no choice - in which case you should really think several times about what it is that you are actually doing.

In all other cases you should bust your boiler trying to use custom readObject()/writeObject() and/or writeReplace()/readResolve() methods and/or serialFields annotations so that you can continue to read objects from those existing serializations. Once you break that you are in for a major headache, indeed nightmare.