In a scenario where an a/c due to operate A to B goes sick, it can happen that for logistical reasons another from the fleet due to operate A to C (around the same time) is re-scheduled to operate the A to B sector. It does not involve any change of crews. Ground staff may have to transfer bags already loaded from one a/c to the other. Both flight plans would have been filed so it's only a matter of cancelling the one no longer operating and, if applicable, the a/c registration on the other. Pax compensation might be cheaper cancelling A to B than A to C, which may well be part of the decision process.