How will I be notified if the aircraft for my flight changes?

A few weeks ago I arrived at the Phoenix airport for a flight home to Baltimore only to discover that the boarding pass I’d downloaded the night before was no longer valid. The aircraft being used for the flight had been changed and everyone had to be reaccommodated. In my case my colleague and I lost our premium economy seats (fee was refunded) and spent the flight in coach. I was in an aisle seat and my colleague in a window a few rows back. I’m reminded of this because this morning I received a question from a client who had the same experience and wanted to know how such a thing could happen without notification. 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; 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.