Why do airlines switch aircraft at the last minute and shouldn’t I be notified if this happens?

There are any number of reasons an airline may need to change the aircraft including non-availability of the original plane, requiring a larger or smaller aircraft to match demand on the route, needing to position a particular aircraft for an ongoing flight, etc. When this is done well in advance the airline will usually notify travel agencies and/or travelers; however, when the change occurs relatively last minute there is no notification until check-in. The airlines reaccommodate passengers as nearly as possible to what they originally had (aisle, window, exit, premium economy, first, etc.), but as the two aircraft will never be exactly the same it is inevitable that someone will be disappointed. Rest assured that if the airline cannot accommodate you in a service you paid a premium for, such as premium economy or special seating, the additional cost will be refunded.