From the page linked above:
may be involuntary rerouted to ... flights in lowest available booking class in same service class as originally booked
One would think that they would offer to rebook into a booking class permitting at least the same flexibility as the booking originally paid for. If I had paid a premium to get, for example, a changeable or extendable ticket and were involuntarily re-routed losing the flexibility I would not be very happy...