Booked today 3/19 on UA 932 DEN-IAD on a domestic-config 767 cattle car, with seat 3F. Tried to check in online at 6:30am, unable to complete, but I saw "757" before I cancelled the process.

Tried to check in at 7:30am, succeeded, with a boarding pass for seat 2B on a 757.

Got to the airport, checked in again at a terminal and this time got seat 3F again.

Gate agent said it was downgauged, then later upgauged. In fact the aircraft was ferried in--nobody got off when it arrived. She tore up my boarding pass for 2B.

I told her others might have checked in online for seats that didn't exist. She said she looked at the passenger list and didn't think so. I should have bet her a nickel. Several people successfully boarded and then found that their boarding passes were for seats that don't exist on a domestic 767.

Moral of the story---know your seat assignment, and if it changes, pay close attention to why.