How is your data stored? PersistentStore, SDCard or RMS?
I presume PersistentStore.
In which case, the first option is not to change the PersistentStore object, or at least, not change it is a way that means it will be stored differently. So for example, if you PersistenStore object is a Vector, adding an element does not change the struture. But if you add a new int to the class, that changes the way ti is stored.
If you really must change the format of the PerisstentStore Object, then you can create a second, different persisted Object, and have your start up processing convert from the old format to the new.
