Are you talking about updating the foreign keys in the database or the values in your related objects?
The database should either update or reject the changes depending on how you have set up the fk.
I think your objects will need to be re-loaded to reflect any changes the database will have made, I cant remember seeing any code in subsonic that persists the primary key change to affected foreign key values.
I think the idea of changing the PK would be a bit foreign to the Subsonic convention of using non-contextual PKs, so I cant see it being catered for in the model. But, I have been wrong several dozen times before 