My fiance (a 1K) & I had confirmed E+ seats together on a 6/25 flight SFO to IAD. (Going to meet the future in-laws for the first time). We were on the same PNR and waitlisted for upgrades to first class. We checked in on line for our E+ BPs but when we got to the gate right when the plane was boarding, she got called up and handed a first class BP. Her seat next to me in E+ had been reassigned to another passenger.

On the plane I asked the FA why that happened and she said that the plane was originally to be a 777 but was changed to a 757. This was not true at all.

How does this happen if we're on the same PNR? How can we avoid this in the future? (other than not requesting upgrades).