Dear wwh:

I'm with Sparteye here. The word, though rare, is pithy, as she observed. And I think it goes beyond what is interchangeable. Function and fungible have the same latinate root: fungi, to perform. With the question of loyalties changing, there's much more involved that what is interchangeable. The change in loyalty brings out a change in performance of functions, actions, and operations. Death could be a consequence of such changes in operations, but fungibility captures the big picture better than interchangeability. Interchangeable seems too light here. Fungible seems more action-packed.
Just my take,
WW