Can Mereological Sums Change Their Parts?

Many philosophers think not. Many philosophers, in fact, seem to suppose that anyone who raises the question whether mereological sums can change their parts displays thereby a failure to grasp an essential feature of the concept “mereological sum.” It is hard to point to an indisputable example of this in print,[i] but it is a thesis I hear put forward very frequently in conversation (sometimes it is put forward in the form of an incredulous stare after I have said something that implies that mereological sums can change their parts)